My supervillain lair is in a mountain that’s on fire (because underwater lairs are too easily infiltrated). Specifically, it’s Yanar Dag in Azerbaijan, on the Caspian Sea.

I feel comfortable giving you the exact location because… well, it’s a mountain. That’s on fire. Please, try to break in.

It’s a natural gas fire that’s never been put out. And just for fun, occasionally, ‘flames jet out into the air 10 feet (3.0 m).’