As the world grows smaller and the luxury hotel scene expands, it's becoming more and more difficult to find a corner of this planet to call your own — not to mention finding a hotel that doesn't look like a thousand others. That said, it's not impossible, as our list of extreme hotels proves. In fact, we had a hard time choosing our favorites and even had to leave out some real treasures, including a sewer pipe hotel on the Danube River and a tree house hotel just 90 minutes outside Paris. But we think you'll agree that our selectivity has paid off. By vacationing at one of the hotels in this collection, you're sure to have an experience that takes you to the limit and beyond.