The back cover reads:
Deep down, at the bottom of the ocean, there are a lot of very strange animals. Like these worms, for instance. They're 12 feet long...and they live in 12-feet-long tubes!
Alvin is a special submarine that explores the ocean floor. Usually it can take only three scientists. But on this trip, yuo get to go along, too. The ocean floor is a beautiful and very different world.