"More than 20 rough-toothed dolphins have died since Wednesday's beaching by about 70 of the marine mammals, Florida Keys National Marine Sanctuary spokeswoman Cheva Heck said Saturday.

A day before the dolphins swam ashore, the USS Philadelphia had conducted exercises with Navy SEALs off Key West, about 45 miles from Marathon, where the dolphins became stranded.

Some scientists surmise that loud bursts of sonar, which can be heard for miles in the water, may disorient or scare marine mammals, causing them to surface too quickly and suffer the equivalent of what divers know as the bends — when sudden decompression forms nitrogen bubbles in tissue."