diff --git a/data/json/mapgen/earth.json b/data/json/mapgen/earth.json
index 05fe844461843..9d67e46799d47 100644
--- a/data/json/mapgen/earth.json
+++ b/data/json/mapgen/earth.json
@@ -1,72 +1,20 @@
 [
   {
+    "type": "mapgen",
     "method": "json",
-    "object": {
-      "rows": [
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................"
-      ],
-      "terrain": { ".": "t_rock" }
-    },
-    "om_terrain": "deep_rock",
+    "om_terrain": "open_air",
+    "object": { "fill_ter": "t_open_air" }
+  },
+  {
     "type": "mapgen",
-    "weight": 100
+    "method": "json",
+    "om_terrain": [ "deep_rock", "empty_rock" ],
+    "object": { "fill_ter": "t_rock" }
   },
   {
+    "type": "mapgen",
     "method": "json",
-    "object": {
-      "rows": [
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................",
-        "........................"
-      ],
-      "terrain": { ".": "t_soil" }
-    },
     "om_terrain": "solid_earth",
-    "type": "mapgen",
-    "weight": 100
+    "object": { "fill_ter": "t_soil" }
   }
 ]
diff --git a/data/json/obsoletion_and_migration_0.I/obsolete_overmap_terrain.json b/data/json/obsoletion_and_migration_0.I/obsolete_overmap_terrain.json
index e7b3fa62eb3e8..e78d40a29f607 100644
--- a/data/json/obsoletion_and_migration_0.I/obsolete_overmap_terrain.json
+++ b/data/json/obsoletion_and_migration_0.I/obsolete_overmap_terrain.json
@@ -183,6 +183,11 @@
       "cabin_liam_driveway_north": "rural_road_forest_ns"
     }
   },
+  {
+    "type": "oter_id_migration",
+    "//": "migrated in 0.I, were stopped from spawning in 2019 but never obsoleted/migrated so just adding this to be safe",
+    "oter_ids": { "hellmouth": "empty_rock", "rift": "empty_rock" }
+  },
   {
     "type": "oter_id_migration",
     "//": "migrated in 0.I",
diff --git a/data/json/overmap/overmap_terrain/overmap_terrain_hardcoded.json b/data/json/overmap/overmap_terrain/overmap_terrain_hardcoded.json
index ac4afd678ee40..528b77993acbb 100644
--- a/data/json/overmap/overmap_terrain/overmap_terrain_hardcoded.json
+++ b/data/json/overmap/overmap_terrain/overmap_terrain_hardcoded.json
@@ -148,7 +148,7 @@
   {
     "type": "overmap_terrain",
     "id": "rock",
-    "//": "This is old rock type, new one (below) will replace it in new overmaps",
+    "//": "This is old rock type replaced by empty_rock/deep_rock, now only used for edges of slimepits",
     "name": "solid rock",
     "looks_like": "empty_rock",
     "sym": "%",
@@ -166,24 +166,6 @@
     "travel_cost_type": "impassable",
     "flags": [ "NO_ROTATE" ]
   },
-  {
-    "type": "overmap_terrain",
-    "id": "rift",
-    "name": "rift",
-    "sym": "^",
-    "color": "red",
-    "see_cost": 2,
-    "flags": [ "NO_ROTATE", "RISK_HIGH", "SHOULD_NOT_SPAWN" ]
-  },
-  {
-    "type": "overmap_terrain",
-    "id": "hellmouth",
-    "name": "hellmouth",
-    "sym": "^",
-    "color": "light_red",
-    "see_cost": 2,
-    "flags": [ "KNOWN_DOWN", "NO_ROTATE", "RISK_HIGH", "SHOULD_NOT_SPAWN" ]
-  },
   {
     "type": "overmap_terrain",
     "id": "tutorial",
diff --git a/data/mods/Graphical_Overmap/go_overmap_terrain_hardcoded.json b/data/mods/Graphical_Overmap/go_overmap_terrain_hardcoded.json
index a747bd4f2cbaa..b0560af45868c 100644
--- a/data/mods/Graphical_Overmap/go_overmap_terrain_hardcoded.json
+++ b/data/mods/Graphical_Overmap/go_overmap_terrain_hardcoded.json
@@ -166,18 +166,6 @@
     "copy-from": "empty_rock",
     "sym": "\u00AE"
   },
-  {
-    "type": "overmap_terrain",
-    "id": "rift",
-    "copy-from": "rift",
-    "sym": "\u00A1"
-  },
-  {
-    "type": "overmap_terrain",
-    "id": "hellmouth",
-    "copy-from": "hellmouth",
-    "sym": "\u00A1"
-  },
   {
     "type": "overmap_terrain",
     "id": "ants",
diff --git a/data/mods/alt_map_key/overmap_terrain.json b/data/mods/alt_map_key/overmap_terrain.json
index 14d9b4c59d6fc..dba29b8265404 100644
--- a/data/mods/alt_map_key/overmap_terrain.json
+++ b/data/mods/alt_map_key/overmap_terrain.json
@@ -2113,22 +2113,6 @@
     "sym": "%",
     "color": "dark_gray"
   },
-  {
-    "type": "overmap_terrain",
-    "id": "rift",
-    "copy-from": "rift",
-    "name": "rift",
-    "sym": "^",
-    "color": "red"
-  },
-  {
-    "type": "overmap_terrain",
-    "id": "hellmouth",
-    "copy-from": "hellmouth",
-    "name": "hellmouth",
-    "sym": "^",
-    "color": "light_red"
-  },
   {
     "type": "overmap_terrain",
     "id": "sewer_sub_station",
diff --git a/src/mapgen_functions.cpp b/src/mapgen_functions.cpp
index 08911be6c6b5c..cee7864570f38 100644
--- a/src/mapgen_functions.cpp
+++ b/src/mapgen_functions.cpp
@@ -37,8 +37,6 @@
 #include "weighted_list.h"
 #include "creature_tracker.h"
 
-static const oter_str_id oter_hellmouth( "hellmouth" );
-static const oter_str_id oter_rift( "rift" );
 static const oter_str_id oter_river_c_not_nw( "river_c_not_nw" );
 static const oter_str_id oter_river_c_not_se( "river_c_not_se" );
 static const oter_str_id oter_river_c_not_sw( "river_c_not_sw" );
@@ -56,7 +54,6 @@ static const ter_str_id ter_t_buffer_stop( "t_buffer_stop" );
 static const ter_str_id ter_t_clay( "t_clay" );
 static const ter_str_id ter_t_dirt( "t_dirt" );
 static const ter_str_id ter_t_grass( "t_grass" );
-static const ter_str_id ter_t_lava( "t_lava" );
 static const ter_str_id ter_t_open_air( "t_open_air" );
 static const ter_str_id ter_t_railroad_rubble( "t_railroad_rubble" );
 static const ter_str_id ter_t_railroad_tie( "t_railroad_tie" );
@@ -67,7 +64,6 @@ static const ter_str_id ter_t_railroad_track_on_tie( "t_railroad_track_on_tie" )
 static const ter_str_id ter_t_rock( "t_rock" );
 static const ter_str_id ter_t_rock_floor( "t_rock_floor" );
 static const ter_str_id ter_t_sand( "t_sand" );
-static const ter_str_id ter_t_slope_down( "t_slope_down" );
 static const ter_str_id ter_t_swater_dp( "t_swater_dp" );
 static const ter_str_id ter_t_swater_sh( "t_swater_sh" );
 static const ter_str_id ter_t_swater_surf( "t_swater_surf" );
@@ -115,18 +111,12 @@ tripoint rotate_point( const tripoint &p, int rotations )
 building_gen_pointer get_mapgen_cfunction( const std::string &ident )
 {
     static const std::map<std::string, building_gen_pointer> pointers = { {
-            { "null",             &mapgen_null },
             { "forest",           &mapgen_forest },
             { "river_center", &mapgen_river_center },
             { "river_curved_not", &mapgen_river_curved_not },
             { "river_straight",   &mapgen_river_straight },
             { "river_curved",     &mapgen_river_curved },
-            { "open_air", &mapgen_open_air },
-            { "rift", &mapgen_rift },
-            { "hellmouth", &mapgen_hellmouth },
-            // New rock function - should be default, but isn't yet for compatibility reasons (old overmaps)
-            { "empty_rock", &mapgen_rock },
-            // Old rock behavior, for compatibility and near caverns and slime pits
+            // Old rock behavior, only used around slime pits
             { "rock", &mapgen_rock_partial },
 
             { "subway_straight",    &mapgen_subway },
@@ -161,21 +151,6 @@ ter_str_id clay_or_sand()
     return ter_t_clay;
 }
 
-/////////////////////////////////////////////////////////////////////////////////////////////////
-///// builtin terrain-specific mapgen functions. big multi-overmap-tile terrains are located in
-///// mapgen_functions_big.cpp
-
-void mapgen_null( mapgendata &dat )
-{
-    debugmsg( "Generating null terrain, please report this as a bug" );
-    for( int i = 0; i < SEEX * 2; i++ ) {
-        for( int j = 0; j < SEEY * 2; j++ ) {
-            dat.m.ter_set( point( i, j ), ter_str_id::NULL_ID() );
-            dat.m.set_radiation( point( i, j ), 0 );
-        }
-    }
-}
-
 int terrain_type_to_nesw_array( oter_id terrain_type, std::array<bool, 4> &array )
 {
     // count and mark which directions the road goes
@@ -216,6 +191,9 @@ void nesw_array_rotate( std::array<T, N> &array, size_t dist )
     }
 }
 
+/////////////////////////////////////////////////////////////////////////////////////////////////
+///// builtin terrain-specific mapgen functions.
+
 void mapgen_subway( mapgendata &dat )
 {
     map *const m = &dat.m;
@@ -702,173 +680,6 @@ void mapgen_rock_partial( mapgendata &dat )
     }
 }
 
-void mapgen_rock( mapgendata &dat )
-{
-    fill_background( &dat.m, ter_t_rock );
-}
-
-void mapgen_open_air( mapgendata &dat )
-{
-    fill_background( &dat.m, ter_t_open_air );
-}
-
-void mapgen_rift( mapgendata &dat )
-{
-    map *const m = &dat.m;
-
-    if( dat.north() != oter_rift && dat.north() != oter_hellmouth ) {
-        if( connects_to( dat.north(), 2 ) ) {
-            dat.n_fac = rng( -6, -2 );
-        } else {
-            dat.n_fac = rng( 2, 6 );
-        }
-    }
-    if( dat.east() != oter_rift && dat.east() != oter_hellmouth ) {
-        if( connects_to( dat.east(), 3 ) ) {
-            dat.e_fac = rng( -6, -2 );
-        } else {
-            dat.e_fac = rng( 2, 6 );
-        }
-    }
-    if( dat.south() != oter_rift && dat.south() != oter_hellmouth ) {
-        if( connects_to( dat.south(), 0 ) ) {
-            dat.s_fac = rng( -6, -2 );
-        } else {
-            dat.s_fac = rng( 2, 6 );
-        }
-    }
-    if( dat.west() != oter_rift && dat.west() != oter_hellmouth ) {
-        if( connects_to( dat.west(), 1 ) ) {
-            dat.w_fac = rng( -6, -2 );
-        } else {
-            dat.w_fac = rng( 2, 6 );
-        }
-    }
-    // Negative *_fac values indicate rock floor connection, otherwise solid rock
-    // Of course, if we connect to a rift, *_fac = 0, and thus lava extends all the
-    //  way.
-    for( int i = 0; i < SEEX * 2; i++ ) {
-        for( int j = 0; j < SEEY * 2; j++ ) {
-            if( ( dat.n_fac < 0 && j < dat.n_fac * -1 ) || ( dat.s_fac < 0 && j >= SEEY * 2 - dat.s_fac ) ||
-                ( dat.w_fac < 0 && i < dat.w_fac * -1 ) || ( dat.e_fac < 0 && i >= SEEX * 2 - dat.e_fac ) ) {
-                m->ter_set( point( i, j ), ter_t_rock_floor );
-            } else if( j < dat.n_fac || j >= SEEY * 2 - dat.s_fac ||
-                       i < dat.w_fac || i >= SEEX * 2 - dat.e_fac ) {
-                m->ter_set( point( i, j ), ter_t_rock );
-            } else {
-                m->ter_set( point( i, j ), ter_t_lava );
-            }
-        }
-    }
-
-}
-
-void mapgen_hellmouth( mapgendata &dat )
-{
-    map *const m = &dat.m;
-    // what is this, doom?
-    // .. seriously, though...
-    for( int i = 0; i < 4; i++ ) {
-        if( dat.t_nesw[i] != oter_rift && dat.t_nesw[i] != oter_hellmouth ) {
-            dat.dir( i ) = 6;
-        }
-    }
-
-    for( int i = 0; i < SEEX * 2; i++ ) {
-        for( int j = 0; j < SEEY * 2; j++ ) {
-            if( j < dat.n_fac || j >= SEEY * 2 - dat.s_fac || i < dat.w_fac || i >= SEEX * 2 - dat.e_fac ||
-                ( i >= 6 && i < SEEX * 2 - 6 && j >= 6 && j < SEEY * 2 - 6 ) ) {
-                m->ter_set( point( i, j ), ter_t_rock_floor );
-            } else {
-                m->ter_set( point( i, j ), ter_t_lava );
-            }
-            if( i >= SEEX - 1 && i <= SEEX && j >= SEEY - 1 && j <= SEEY ) {
-                m->ter_set( point( i, j ), ter_t_slope_down );
-            }
-        }
-    }
-    switch( rng( 0, 4 ) ) { // Randomly chosen "altar" design
-        case 0:
-            for( int i = 7; i <= 16; i += 3 ) {
-                m->ter_set( point( i, 6 ), ter_t_rock );
-                m->ter_set( point( i, 17 ), ter_t_rock );
-                m->ter_set( point( 6, i ), ter_t_rock );
-                m->ter_set( point( 17, i ), ter_t_rock );
-                if( i > 7 && i < 16 ) {
-                    m->ter_set( point( i, 10 ), ter_t_rock );
-                    m->ter_set( point( i, 13 ), ter_t_rock );
-                } else {
-                    m->ter_set( point( i - 1, 6 ), ter_t_rock );
-                    m->ter_set( point( i - 1, 10 ), ter_t_rock );
-                    m->ter_set( point( i - 1, 13 ), ter_t_rock );
-                    m->ter_set( point( i - 1, 17 ), ter_t_rock );
-                }
-            }
-            break;
-        case 1:
-            for( int i = 6; i < 11; i++ ) {
-                m->ter_set( point( i, i ), ter_t_lava );
-                m->ter_set( point( SEEX * 2 - 1 - i, i ), ter_t_lava );
-                m->ter_set( point( i, SEEY * 2 - 1 - i ), ter_t_lava );
-                m->ter_set( point( SEEX * 2 - 1 - i, SEEY * 2 - 1 - i ), ter_t_lava );
-                if( i < 10 ) {
-                    m->ter_set( point( i + 1, i ), ter_t_lava );
-                    m->ter_set( point( SEEX * 2 - i, i ), ter_t_lava );
-                    m->ter_set( point( i + 1, SEEY * 2 - 1 - i ), ter_t_lava );
-                    m->ter_set( point( SEEX * 2 - i, SEEY * 2 - 1 - i ), ter_t_lava );
-
-                    m->ter_set( point( i, i + 1 ), ter_t_lava );
-                    m->ter_set( point( SEEX * 2 - 1 - i, i + 1 ), ter_t_lava );
-                    m->ter_set( point( i, SEEY * 2 - i ), ter_t_lava );
-                    m->ter_set( point( SEEX * 2 - 1 - i, SEEY * 2 - i ), ter_t_lava );
-                }
-                if( i < 9 ) {
-                    m->ter_set( point( i + 2, i ), ter_t_rock );
-                    m->ter_set( point( SEEX * 2 - i + 1, i ), ter_t_rock );
-                    m->ter_set( point( i + 2, SEEY * 2 - 1 - i ), ter_t_rock );
-                    m->ter_set( point( SEEX * 2 - i + 1, SEEY * 2 - 1 - i ), ter_t_rock );
-
-                    m->ter_set( point( i, i + 2 ), ter_t_rock );
-                    m->ter_set( point( SEEX * 2 - 1 - i, i + 2 ), ter_t_rock );
-                    m->ter_set( point( i, SEEY * 2 - i + 1 ), ter_t_rock );
-                    m->ter_set( point( SEEX * 2 - 1 - i, SEEY * 2 - i + 1 ), ter_t_rock );
-                }
-            }
-            break;
-        case 2:
-            for( int i = 7; i < 17; i++ ) {
-                m->ter_set( point( i, 6 ), ter_t_rock );
-                m->ter_set( point( 6, i ), ter_t_rock );
-                m->ter_set( point( i, 17 ), ter_t_rock );
-                m->ter_set( point( 17, i ), ter_t_rock );
-                if( i != 7 && i != 16 && i != 11 && i != 12 ) {
-                    m->ter_set( point( i, 8 ), ter_t_rock );
-                    m->ter_set( point( 8, i ), ter_t_rock );
-                    m->ter_set( point( i, 15 ), ter_t_rock );
-                    m->ter_set( point( 15, i ), ter_t_rock );
-                }
-                if( i == 11 || i == 12 ) {
-                    m->ter_set( point( i, 10 ), ter_t_rock );
-                    m->ter_set( point( 10, i ), ter_t_rock );
-                    m->ter_set( point( i, 13 ), ter_t_rock );
-                    m->ter_set( point( 13, i ), ter_t_rock );
-                }
-            }
-            break;
-        case 3:
-            for( int i = 6; i < 11; i++ ) {
-                for( int j = 6; j < 11; j++ ) {
-                    m->ter_set( point( i, j ), ter_t_lava );
-                    m->ter_set( point( SEEX * 2 - 1 - i, j ), ter_t_lava );
-                    m->ter_set( point( i, SEEY * 2 - 1 - j ), ter_t_lava );
-                    m->ter_set( point( SEEX * 2 - 1 - i, SEEY * 2 - 1 - j ), ter_t_lava );
-                }
-            }
-            break;
-    }
-
-}
-
 void mapgen_forest( mapgendata &dat )
 {
     map *const m = &dat.m;
diff --git a/src/mapgen_functions.h b/src/mapgen_functions.h
index 0c4b0939a88f6..76908852b426c 100644
--- a/src/mapgen_functions.h
+++ b/src/mapgen_functions.h
@@ -37,19 +37,12 @@ ter_str_id grass_or_dirt();
 ter_str_id clay_or_sand();
 
 // helper functions for mapgen.cpp, so that we can avoid having a massive switch statement (sorta)
-void mapgen_null( mapgendata &dat );
 void mapgen_forest( mapgendata &dat );
 void mapgen_river_center( mapgendata &dat );
 void mapgen_river_curved_not( mapgendata &dat );
 void mapgen_river_straight( mapgendata &dat );
 void mapgen_river_curved( mapgendata &dat );
-void mapgen_cave( mapgendata &dat );
-void mapgen_cave_rat( mapgendata &dat );
-void mapgen_rock( mapgendata &dat );
 void mapgen_rock_partial( mapgendata &dat );
-void mapgen_open_air( mapgendata &dat );
-void mapgen_rift( mapgendata &dat );
-void mapgen_hellmouth( mapgendata &dat );
 void mapgen_subway( mapgendata &dat );
 void mapgen_lake_shore( mapgendata &dat );
 void mapgen_ocean_shore( mapgendata &dat );