When legions of hammerhead sharks pass over filmmaker Howard Hall, the schools are so dense they blot out the sun. From Shark Mountain, airing on the NATURE series, July 29th at 8 p.m. (ET) on PBS (check local listings).

The film takes viewers to Cocos Island in the Pacific, where sharks of all kinds converge in staggering numbers.

For more information, visit http://www.pbs.org/wnet/nature/sharkmountain/index.html