There are plenty of artsy, hip and techno-fabulous hotels around the world. New Zealand's Hapuku Tree Houses, Turkey's Gamirasu Cave Hotel, the Hobbit Motel. That's not what we've collected here. These are the overnight stays that are just plain strange.

Enter the world's 12 weirdest hotel rooms, after the jump.

Propeller Island City Lodge, Berlin

Propeller Island's collection of chambers are more like psychological experiments than holiday retreats. Although it's hard to pick just one, the upside-down room has all of its furniture nailed to the ceiling. (The Lodge's kaleidoscope room has its charms as well.) $165/night

Jumbo Hostel, Stockholm, Sweden

While the stay-in-an-airplane concept doesn't quite pass our weirdness test, Sweden's Jumbo Hostel gets bonus points for actually dividing the airplane into different classes of accommodation. We say go for the cockpit. If you can't seal the deal there, you might as well give up. $460/night, with breakfast.

Sala Silvermine , Sweden

We were about to highlight the Kokopelli Cave in New Mexico when we came across Sweden's Sala Silvermine offering a suite 155 meters underground. (Leave your cell phone at home, their site advises.) The hotel "pool"? Deeper caverns accessible only by scuba diving. $520/night.

Jules' Undersea Lodge, Key Largo, Fla.
This is actually not the world's only underwater hotel. (A far more lavish underwater resort is in the planning stages in, surprise, Dubai.) This former research lab, which you must be a certified scuba diver to enjoy, does offer a great honeymooner's package and a reasonable "day rate." At least you won't have to worry about the maid walking in. $375 pp/night, $250 per couple/3-hour day rate.