Mr. Mine Wiki

Bosses are unique enemies that appear from depth 400km to the bottom of the earth every 100km, each one you need to kill before your drill can keep going and you if you kill the first three you can complete quest 21 in the underground city.

The first boss is the Imp unerworld found at 400km

The second is the [trash/garbage/???]....

There also are moon bosses, ...

for more information go to the Monsters page.