Exley was also one of the first divers to introduce Trimix to cave diving. While early experiments using mixed gases in the U.S. had tragic outcomes (Exley's friend Louis Holtzendorf died on one such dive), Exley's deep dives at Nacimiento del Rio Mante, a Mexican cave or cenote, proved the usefulness of Trimix for cave diving. Not only could these mixtures allow a diver to go deeper without succumbing to narcosis or oxygen poisoning, but they also reduced the amount of time spent at decompression stops during the ascent. In March of 1989, he descended to a depth of 881 feet using Trimix, a world record at the time. He returned to the surface after 14 hours of decompression with no side effects.
In August of 1993, Exley reached 863 feet when he touched bottom in Bushmansgat (Bushman's Hole) in South Africa, but not before experiencing a serious case of high pressure nervous syndrome (HPNS) that included blurred vision and intense, uncontrollable tremors. He joined Jim Bowden in focusing his efforts on a cave known as Zacatón (aka Pit 6350) just north of Tampico, Mexico. The cave is known to be at least 1,080 feet deep. In September, Bowden dove to 774 feet, Ann Kristovich, the team physician, reached 541 feet during a dive, a new depth record for women. The previous record had been set at Rio Mante by Mary Ellen Eckoff, Exley's wife.
Sheck Exley died at age 45 on April 6, 1994 while attempting to dive to a depth of over 300 meters (1,000 ft) in Zacatón. He was about 10 meters away from the bottom of the cave. He made the dive as part of a dual dive with Jim Bowden, but Bowden aborted his descent early when his gas supply ran low. Exley's body was recovered only because he had hooked his arms in the descent line, perhaps to sort out gas issues. His wrist-mounted dive computer did show a maximum depth of 268 meters (879 ft). It is not certain what caused his death; team members concluded the causes "...could include stress of HPNS exacerbated by the narcotic effects of nitrogen at that depth".
He did write two major books on the subject: Basic Cave Diving: A Blueprint for Survival and Caverns Measureless to Man. Furthermore, he established many of the basic safety procedures used today by cave divers around the world. Exley was also a pioneer of extreme deep water diving. In the book, Diving into Darkness (a story about Dave Shaw and Don Shirley) it was commented: "Exley's status in the sport is almost impossible to overstate."
A Facebook site not only remembers Exley’s achievements, it also provides a discussion forum for over 2,300 fans of the biggest cave divers of all times.