A blue hole is a flooded cave that formed in the limestone rock thousands of years ago, when the sea level was much lower and rainwater slowly carved tunnels underground. When the sea rose, these caves filled with water and became the deep, round pools we see today. From above, they glow a much darker, richer blue than the shallow water around them.
Some blue holes are in the ocean, while others — called inland blue holes — are found on the land of Andros island itself, surrounded by trees. You can find them by looking for a sudden dark circle among the green vegetation or the pale sandy shallows. Scientists love them because the layers of water inside can be like a time capsule, holding clues about what the Earth was like long ago.
Inside a blue hole, the deeper you go, the darker and more mysterious it gets. Marine life including fish, sponges, and crustaceans live near the opening where sunlight still reaches. The caves twist and branch in amazing directions underground. Trained divers who explore them sometimes discover completely new tunnels that no human has ever seen before.
The Andros Barrier Reef, just offshore, is the third-largest barrier reef in the world. Together with the blue holes, it makes Andros an underwater wonderland. Local Bahamians sometimes call the blue holes magical, and it is easy to see why — they look like secret portals into another world.