Cave diving is truly an extreme pursuit. Unlike most sports, as cave diving matures and gets more mainstream, cave divers grow more daring than at any previous point in the sport’s history. Also, as new caves are discovered, longer, deeper and more dangerous challenges are turning up.

Here are four of the most extreme cave diving spots on earth.

The Samaesan Hole 

To the southeast of Samae San, or Samaesan Bay in Thailand (shown in the video above) there exists a hole in the sea shelf that falls nearly 300 feet to the pitch black sea floor. “The Hole” is the deepest dive in the Gulf of Thailand — a dive recommended for “experts only” who are TRIMIX certified and able to withstand currents that can rip a diver into the open sea. More dangerous may be the tankers that plow through the shipping lanes abutting the Samaesan Hole, not to mention the unexploded ordinance that rests tantalizingly on the sea bed. The Samaesan Hole isn’t just a great place to shine a light on deep sea wildlife, it’s also a military dump site.

Pozo Azul 

In September 2010, underwater explorers Jason Mallinson, Rick Stanton, John Volanthen and Rene Houben accomplished the longest cave dive exploration ever, swimming 5.5 miles into the heart of the near-freezing freshwater Pozo Azul caves in northern Spain. Unlike most cave divers, they breathed fresh air during their voyage into the unknown, surfacing in a dry cave where they spent two nights replenishing supplies provided by a network of support divers who ferried goods — mostly air — through the silt-clouded catacombs. This 50-hour dive was a logistical nightmare and a real group effort completed by what might be the largest dive team ever amassed.

The Blue Holes of Andros & Abaco 

No place on earth offers holes into the unknown like the Bahamas. Andros, the country’s largest island, is home to more than 220 blue holes all by itself. Like nearly all blue holes on earth, the blue holes of the Bahamas formed during past ice ages when the oceans were locked up in massive glaciers and sea level was more than 300 feet lower than it is today. As the ice melted, the water came pouring in, forming caverns that plummet beyond the sun’s reach and snake under the island’s shores. Today, advanced divers navigate these blue holes, stringing line behind them so they can make their way out again should tanks run low or divers become disoriented.

Wakulla Springs – Florida

The preeminent cave diving destination in the United States, Wakulla Springs, located just south of Tallahassee, Florida, might be the longest and deepest submerged freshwater cave system in the world. Wakulla Springs offers a cornucopia of diving destinations that will easily kill you if you’re unprepared. Jarrod Jablonski and Casey McKinlay were not unprepared. In 2007, the duo swam continually for more than 20 hours, covering seven miles of cave floor from one entrance of the system to another, with each diver exhausting at least 20 full scuba cylinders. Today, the dive stands as the longest ever entrance-to-exit cave dive ever completed — an accomplishment the team never would have achieved had they not laid miles of guide line over the previous decade.

Cover Photo Credit: DiveKarma / Flickr.com