On the 6th of April, 1994, experienced cave divers Jim Bowden and Sheck Exley entered
the crisp blue waters of the Zacatón Sinkhole in northeast Mexico.
They were there to attempt a record-breaking dive to the very bottom of one of the deepest
flooded sinkholes in the world.
It was an attempt that had been months if not years in the planning, and it was an attempt
from which – unfortunately – only one of them would return alive.
Sheck Exley started diving at the age of sixteen.
He undertook his first ever cave dive that same year, and it would be an experience that
would forever change his life.
From that point on he was hooked on cave diving, and dedicated the majority of his time to
honing his skills, increasing his knowledge, and planning and executing ever-more-adventurous
diving expeditions.
Sheck was the first person in recorded history to log more than 100 cave dives – a feat
he managed by the time he turned 23.
During the long career that followed, he would go on to log a total of 4,000 dives, many
of them technical or cave dives.
His extensive experience lead him to write two books on the subject, and also to become
the first ever chairman of the Cave Diving Section of the American National Speleological
Society.
He was also responsible for several innovations in safety that benefitted all cave divers
– including pioneering the practise of having to hand a second, backup regulator.
In his personal life Sheck was greatly admired.
He was described as calm, quiet and mild-mannered by many people who met him.
After working for a time in a car dealership, he settled on a career teaching mathematics
at a high school in Florida.
The job allowed him to travel at the weekends, and he did so often… but also spent a great
deal of time on the children he educated.
He was popular with his students, and often allowed them to call him on the phone if they
needed help with their homework.
He was respected and admired both as a teacher, and by the cave diving community.
As well as his intense dedication to cave diving, Sheck’s personal physiology also
helped him become the prolific diver he was.
Over the course of many dives it became apparent that he was unusually resistant to nitrogen
narcosis – a condition that causes temporary confusion and a loss of co-ordination in divers
who descend to great depths.
Sheck’s career, long as it was, was not without incident.
In 1968 Sheck, then nineteen, and his brother Edward, sixteen, were freediving at Wakulla
Springs in Florida.
The pair were testing their limits to see who could dive the deepest on a single breath
hold.
No diving equipment was allowed at Wakulla Springs, but the two brothers – with some
practise – could dive to depths of 15 meters (or 50 feet) with nothing more than the air
in their lungs.
During one such dive, Edward ran out of air before he could return to the surface.
Sheck, already exhausted from repeated freedives, ran to his car to retrieve an oxygen tank
he had stored there… but collapsed before he could use it to try and rescue his brother.
Another diver who happened to be present was able to use the tank to retrieve Edward, who
passed away later that day in hospital.
The death of his brother was a terrible blow for Sheck, and fuelled his desire for greater
safety in diving.
He later noted that his passion for cave diving ran so deep that he was unable to give it
up… but that he was desperate to make the practise safer for all those who undertook
it.
A few years later, in December 1970, Sheck was working as a safety diver in the Bahamas.
Two other divers were attempting to set the record for the deepest dive using only compressed
air.
Divers who travel to great depths rarely use only compressed air, as the mixture of gases
therein can cause severe medical issues when breathed under pressure.
Nitrogen narcosis and oxygen toxicity are just two of the risks that have to be considered
when diving using air and not other gases.
During this dive, Sheck reached a depth of 142 metres (or 465 feet) before starting to
suffer confusion, dizziness and the beginnings of a blackout.
With his unusual resilience, he was able to safely return to the surface.
The two divers who were attempting to break the depth record had, however, gone deeper.
They were unable to return and passed away under the water.
This experience left Sheck with a general scepticism for big high-profile dives with
commercial sponsorships and media coverage.
He repeatedly espoused his view that it was better to be cautious, avoid outside pressure
to succeed, and be ready to abort a dive at a moment’s notice if something appeared
to be going wrong.
On the 6th of April 1994, Sheck Exley and a fellow diver named Jim Bowden attempted
a dive at the Zacatón Sinkhole in Mexico.
They were joined by several supporters and safety divers who would prepare equipment,
monitor their dives and generally assist in the attempt.
The sinkhole, or cenote, is a deep, vertical shaft surrounded at the surface by sheer cliffs.
On the surface of the warm, clear water float several islands of zacate grass – it is
from these that the sinkhole gets its name.
Rather than risking the sheer cliff walls above the main pool, Sheck and Jim would enter
the water from a side pool, and swim through a short tunnel to the main pool before beginning
a rapid but carefully controlled descent to the bottom over 300 meters (or 1,000 feet)
below.
They would descend in parallel, following separate weighted lines, spaced a short distance
apart.
As he approached a depth of 268 meters (or 874 feet) Jim checked the amount of gas he
had left in his tank, and noticed with alarm that it was significantly less than he had
anticipated.
He immediately stopped his dive and began to ascend – a process that would take more
than nine hours because of the need to stop regularly and decompress on the way up.
Failure to stop and decompress could cause the bends – a debilitating and sometimes
fatal illness that divers often experience when returning from depth too quickly.
The reason why Jim had less gas available than expected has not been definitively determined,
but could be due to a number of factors.
He may simply have been breathing slightly faster or slightly more deeply than he had
calculated he would.
Even a slight increase in rate of breathing is enough to catastrophically deplete a diver’s
gas reserves.
While Jim ascended slowly to the surface, a safety diver followed Sheck’s line down
to try and determine what had happened to him.
Had he also used up more gas than anticipated?
And if so, why hadn’t he turned around?
The safety diver was unable to see him, but did spot the laminated pages of his dive profile
floating up towards the surface – a definite sign that all was not well.
Less than twenty minutes into the dive, it became clear that Sheck Exley would not return.
His body would be recovered a few days later when the support team returned to remove the
equipment they had put in place for the dive.
It appeared that he had deliberately entangled himself in the line.
Other divers have speculated that he did so when he realised that he was in fatal difficulty,
in order to make the subsequent retrieval of his body easier.
As to exactly what difficulty had ended his life – there were no certain answers.
One likely scenario was that he suffered High Pressure Nervous Syndrome, a disorder caused
by rapid changes in pressure, which can cause convulsions, tremors, nausea, dizziness and
confusion.
These symptoms, while not immediately deadly in and of themselves, might have distracted
Sheck from the fact that he had used more gas than anticipated, resulting in further
complications as the dive progressed.
Equipment failure or a sudden blackout are also possible explanations.
Sheck was laid to rest with a memorial service at the Ortega Methodist Church in Jacksonville.
Jim Bowden, despite suffering some effects of decompression sickness, made a full recovery
and continued to dive for many years.
The Zacatón Sinkhole remains a prominent diving location, and a site of great scientific
interest.
Sheck Exley will never dive there again, but his legacy lives on.
The passion and dedication he displayed during his life made cave diving a safer practise
for all those who follow after him.