Off the coast of Belize exists a giant sinkhole in Earth's ocean. Known as the Great Blue Hole, it is 300 meters (984 feet) across and reaches a depth of 124 meters (407 feet). No light or oxygen is found at the bottom of the hole, so it's not a surprise that it has retained an aura of mystery about itself.
A recent expedition shining a literal light on the hole made several intriguing discoveries about what’s within the deep hole. Where light didn’t reach, the team used sound. They were able to produce the first three-dimensional sonar image of the sinkhole.
“With a variety of sonar heads, used sequentially over the course of the Expedition, we’ve been able to create a high-resolution, three-dimensional sonar image of the interior of the Great Blue Hole,” Erika Bergman, oceanographer and the chief submarine pilot on the expedition, wrote in a blog post on Southern Fried Science.
The team was intrigued by certain targets in particular. Among them, they wanted to study the stalactite caverns at depths between 40 and 50 meters (130 and 160 feet). These formed back when the sea level was over 100 meters (about 330 feet) lower and the Great Blue Hole was a dry cave, about 15,000 years ago. Marine lifeforms have now turned those geological formations into their own home.
The team also observed a layer of calcium carbonate at 88 meters (290 feet), where a barrier reef used to exist. They also looked for the presence of stalactites and stalagmites near the bottom of the hole. They found some evidence of small formations covered in sand, which over the millennia had entered the sinkhole.
“This is also evidenced by the 'conch graveyard,' a stretch of the blue hole where we observed hundreds of dead conch that had presumably fallen into the hole and been unable to escape the steep walls or survive long without oxygen. It was otherworldly down there and our data is one more way to share this revelation,” Bergman explained.
The expedition included some famous names such as billionaire Richard Branson and Fabien Cousteau. Cousteau is the grandson of Jacques Cousteau, co-inventor of the first safe and successful open-circuit scuba.
