Amazing photographs of an underground water system in Japan called the G-Cans Project. Wikipedia Link. Flickr has more.

Some BabelFish translation is in order for the quote.

The national capital region contour sluice may overflow takes in the flood of the small-to-medium-sized rivers which become to the underground, lets flow to Edogawa through the tunnel of the entire extension 6.3‡q which penetrates underground 50 m, it is the worldwide biggest flood prevention facility. It concentrates the leading engineering works technology which Japan is proud in the world, to presently entire approximately 8 tenths complete. The remarkable flood control effect is shown e.g., it works partly from 2002, processes approximately 450 ten thousand t thing floods so far safely.

Clicking around the site leads to some of those interactive 360° panaroma things. Some videos too if you look. Reported on boingboing a long time ago, but rediscovered via creativeireland. Thanks Gordy.