In 2018, business magnate Richard Branson, Fabien Cousteau (Jacques Cousteau’s grandson), and a team of scientists descended into the Great Blue Hole to find out what lies beneath. The expedition made use of two submarines with high-tech equipment so a 3D map of the hole could be captured. Near the bottom of the hole, the team was able to see stalactites, which are common formations on caves and serves as proof that the cave was once not underwater, according to CNN.

Another interesting find was a layer of hydrogen sulfide at about 300 feet into the hole. Per Erika Bergman, an oceanographer who was part of the team, all the light from above diminishes at about that level and everything turns completely dark. The team was also glad to report that the Great Blue Hole is pretty clean and clear, with just a few pieces of plastic here and there.

Despite the Great Blue Hole’s popularity around the world, there are only a few people who have been in its depths, and a few of them never made it out alive. Bergman said that they were able to spot a couple of those people and alerted the Belize government of their location per Business Insider.