1) It technically qualifies you as a hermit. The tax benefits are astounding.
2) All you need to add a new room is water and a lot of time.
3) It's a great place to park your Batmobile.
1) Most of your income will be spent on batteries for your flashlights. That's not deductible.
2) You'll have to explain the meaning of "spelunking" a lot.
3) Your little gay friend will constantly be stopping by to take you out to "fight crime."