This mod will add a few underground structures to the overworld.
Main version:
Structures:
Travel hideout: These structures will give you a great place to get some needed items for you while mining.
These structures can be abandoned having cobweb or without cobwebs and also has a smaller and bigger version.
Chest dungeons: These structures give some good loot but might contain traps.
These structures have a jungle, desert and normal variant.
Spawner dungeons: There are now stray spawners in cold biomes and husk spawners in deserts.
Small shrines: These can be from stone bricks or from nether bricks.
Underground well: This is a small pit with water that generates underground.
Small ore veins: These are small veins of ore that can now be found to make mining more interesting. These veins can contain iron with andesite or gold with andesite or ice with snow blocks.
Version 2:
Fixed bugs:
Structures are now not able to spawn above ground or floating in the ocean.
New structures:
There is an extra chest dungeon.
There are now new shapes of ore veins.
There are now magma and coal veins.
There are now zombie villager and silverfish spawner dungeons.
There are now drowned spawner dungeons that generate on the bottom of oceans.
There are now fake end portal rooms.
There are now end shrines.
There are now small rooms with a bed.
There are now small tunnels that can generate in your world.
Version 3:
Fixed bugs:
the structures can not spawn floating in the air anymore.
new structures:
Obsidian spikes that can only generate in lava.
Ice spikes generating in cold biomes.
Nether Pyramid with a respawn anchor.
Desert tomb generates in deserts and has some loot and traps.
Lava and water 1 block structures to make mindlessly mining more dangerous.
Version 4:
New structures :
The catacomb: A big structure that is dangerous to walk through because of traps and a lot of darkness but has some decent rewards.
Fossil fragments: Smaller pieces of fossils with some coal ore and bone blocks
Deep fossil fragments: Smaller pieces of fossils with some diamond ore and bone blocks generates pretty deep and very rarely.
Fossil huts: Small huts made in pieces of fossils.
Version 5 (1.17.1):
New structures :
Small geodes: geodes that are very small.
Copper, deepslate and dripstone gateways: gateways that can generate from copper, deepslate or dripstone. 
Small calcite and copper ore veins: new veins now with copper ore or being fully calcite.
I have a discord server where you can ask questions, talk or put bug reports if you want to do it there https://discord.gg/bBj5vKGFMw.
This mod has been made with mcreator
