Just Dig

You were messing around with explosives on top of a mountain. The ground gave out.
Now you're inside it, a lot deeper than you'd like. There's no way back up. You do have a shovel.
Start digging.

Dig first

Dig with shovels, pickaxes, drills and explosives. The Tunnel Ray Gun melts straight through rock.
Break through a wall and you'll often find a shop, a fishing spot or a monster nest. If you're lucky, a whole village.
Dig into water and the tunnel floods. Dig into magma and it gets worse. Where water meets magma, it turns to stone.

It gets weird down there

Under the dirt there's a forest. Further down you'll find a coast, sand you sink into, ice caves, boiling magma, and black holes that drop you out somewhere else.
Every area has a boss. Follow its trail to find it.

The locals hate you

Monsters roam in packs and will dig through walls to get to you.
Fight with your shovel, or buy a gun and fit it with a scope, a laser and an extended mag.
Leave the shopkeepers alone. They have shotguns.

Everything you dig up is money

Even dirt drops a few coins, and a gold vein pours them out. Things like a golden toilet sell for a lot.
Spend your coins on a better pickaxe, body upgrades or gear enhancements. Enhancing can fail, and gear that breaks is gone for good.
Fix the buried truck and drive it through the mountain, or buy a mini car that drives on walls and ceilings.
A mole shows you the way, and a corgi brings you jobs.

Getting home

Place the bosses' heads on the altar deep in the Void and the door opens.
Whether that's the only way out is for you to find out.