Gunkan Jima, aka "Battleship Island" is a man-made island, now abandoned. It was built as a coal mine and at it's peak housed 5000 people. It was also the very first place (so I've heard) to have buildings manufactured from poured concrete. It is now used by the University of Nagasaki to study the way concrete deteriorates.
I once swam around this island with two friends. It was scary because there are only two places on the island that you can actually get down to the water. The rest of the island is surrounded by a 10 meter high wall designed to protect it from typhoon generated waves. In any case we started swimming on the inland side where there were no waves and the water was very calm. As we came around the seaward side we encoutered 2-3 foot chop which made swimming really difficult. Unfortunately we had to keep swimming to get back to the access point. I just remember telling myself not to panic or I would be in serious trouble. I was also the weakest swimmer of the three of us so eventually I lost sight of my friends.
We finally made it to safety after swimming for about 2 hours.
The local fisherman who we had hired to take us out to the island had come back to pick us up and fortunately waited for us (he didn't know we were swimming, in fact, he told us NOT to swim because it was dangereous).
When we got back to Nagasaki we were starving and exahusted. We went to some Italian restaurant and ate like 5 orders of pasta.
Photo Tour Wikipedia Entry