Nestled inside a protected cove on the coast of Long Island, Dean’s Blue Hole is an unusual natural wonder. From the shore, it looks like a round patch of blue water surrounded by turquoise green. That’s the result of a massive sinkhole that collapsed near the shore, creating a natural tunnel that drops to more than 660 feet deep, making it the deepest blue hole in the world. The site is commonly visited by scuba divers and free-divers who explore the vertical cavern walls. It’s also a great place for swimmers, and because it’s protected within the cove there are no currents or waves to worry about.