How did Sheck Exley die?

Sheck Exley was in incredible pioneer of cave diving.  On April 6th, 1994, he never surfaced while trying to break an 867 ft record in Mexico's Zacaton Sinkhole.  Exley and his partner Jim Bowden used two separate descent lines and dove at the same time.  They were unaware of the position of one another throughout the dive.  Bowden surfaced after he had reached a depth of 915 ft, making him the new record holder.  When Exley was pulled up, his meter read only 904 ft.