These are stalactites in an under-hang within the Great Blue Hole in Belize. The hole is about 1000 feet across and 400 feet deep. It is much bigger than the Lost Blue Hole in the Bahamas. But, the one in the Bahamas has more marine life around it. The existence of these stalactites indicate that this hole was above water at one point in history. That is the only way they could have formed. It is believed that this was once a vast underground cave system that collapsed when sea levels rose during the last ice age.