Five miles in diameter; 1300m mountains with only one very narrow entry. A perfect place to start your own little satanic doom town.
Except it's in Siberia. It's not inviable -- you'd just need more slaves to compensate for the climate. And who knows, if the place warms up enough you could even grow temperate crops and have a super-long growing period, and if not then just sit back and tax the local reindeer herders nearly to death.
There might be some Russian scientists making radioactive monkeys there right now, but you could just get them to work for you.
I think Google Earth is making me insane...






