One person's dead and a second's missing after a gigantic rock slab apparently broke off a sandstone arch at Lake Powell and smashed onto a boat.

The National Park Service said the slab, approximately 250 square feet in size, fell onto the bow of the boat, capsizing it.

Other boaters discovered the accident Friday, saying they had passed the craft 20 minutes earlier in Lake Canyon and had waved to the two people on board.

Rangers recovered the body of a woman.

Divers were called to the site, where water depths are approximately 150 feet, to find the second victim.

Names of the victims weren't immediately available.