Mother Nature and all the wonders she provides us with never ceases to amaze and inspire. Our globe of land and sea is now about four-and-a-half billion years old, holds a whopping population of over seven billion people, and has gone (and will continue to go) through various phenomenal changes, such as the gradual or sudden formations of natural depressions in the Earth’s surface. Some of the deepest holes in the world have been created in the most unexplainable of ways—both through nature alone and with a little help from mankind. Blue holes are one prime example.

Typically found in low-lying coastal regions, these underwater sinkholes are formed by the erosion of carbonate rocks and appear to the naked eye to look like deep dark blue circles of water in the ocean. The greater absorption of sunlight allows for the dramatic contrasts of aqua-ish color in these not-so-shallow marine caverns, and often times, can seem rather dangerous and frightening to modern man. They are, nonetheless, more than geologically stunning, and further remind us that Mother Earth really does have so much to offer.

The Blue Holes:

1. Glory Hole - Monticello Dam, California

Not only does this one boast a fairly provocative name, but it has a depth of 304 feet, can drain approximately 48,000 cubic feet of water per second, and was purely man-made. Located in Napa County, California, the dam was constructed between 1953 and 1957 as part of the Solano Project, and when at its highest capacity, is definitely not the most fun to swim in! Technically, it’s known as the Morning Glory Spillway, but for locals, it’s known just as good old Glory Hole.

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2. Great Blue Hole - Belize

There may be many blue holes off the coast of Belize, but the greatest of them all is no doubt the "Great" Blue Hole on Lighthouse Reef. The large circular-shaped submarine sinkhole is the most visible in a flyover and the most famous for diving into, especially since it is about 1,000 feet in diameter and 412 feet in depth. Legend says that it was formed as a limestone cave during the last Ice Age over several episodes of quaternary glaciation.

Advanced divers especially love this one, as it allows them to descend to as much as 135 feet to explore exotic wildlife and underwater features. It was even named by the late Jacques Cousteau as one of the best scuba diving areas in the world. So go ahead and take a tour of the lip of the crater. Better yet, dive head on first into this deep blue coral fish-filled underwater heaven. It truly is beautiful.

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3. Dean’s Blue Hole - The Bahamas

Located west of Clarence Town on Long Island, this one is almost twice as deep as any other explored cave in the Bahamas and is basically the deepest saltwater blue hole the world has to offer. At the surface, it is roughly circular, with a diameter ranging from 25 to 35 meters. However, after descending 20 meters, the hole widens considerably into a cavern with a diameter of 100 meters.

Fun fact: In April 2008, the director of Vertical Blue Freediving School and Team, William Trubridge, who often leads dives into Dean's Blue Hole, broke his own freediving world record from 2007, swimming 86 meters (282 feet) into the 660-foot deep hole without using any fins. Two years later in 2010, he went even further by swimming to a depth of 101 meters on a single breath using only his hands and feet for propulsion. Talk about daring.

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4. Blue Hole - Dahab, Egypt

Deep and filled with intricate passageways, this one is known for its superb difficulty. It is a diving space located right on the coast of the Red Sea and is the second deepest blue hole in the world at 130 meters. This danger-ridden coral reef area has claimed the lives of many divers who were attempting to find the tunnel (known as "The Arch") that connects the hole to the open water at about 56 meters in depth. There is a local legend claiming that this east Sinai wonder is cursed by the ghost of a girl who drowned herself in order to escape from an arranged marriage. So be extra careful!

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5. The Blue Hole of Gozo - Maltese Islands

With several depths and routes to choose from, this deep blue void is one of the most popular dive sites in the Maltese Islands and is located just off the west coast of the island of Gozo in Dwejra. Overlooking the beautiful Azure Window-- a towering rock formation that’s geologically phenomenal all on its own—this Mediterranean underwater limestone cave is 15 meters deep, 10 meters wide, and is sandwiched in the bedrock of a cliff. Underwater, though, it’s connected by a 262-foot tunnel to the open sea and features a beautiful array of marine life including octopi, bream, fire worms, and sea horses.

Further, as a natural rock formation, the iconic diving area seems to resemble an upright tube that’s been carved out by centuries of wind and wave power. It can be reached by just a simple walk over some ancient coralline rocks and even offers a sheltered entryway for divers, guiding them through a huge archway starting at 6m and stretching right down to the large cave at the bottom of the hole. In terms of scenery, this one is utterly fantastic and definitely worth exploring.

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6. Watling's Blue Hole - The Bahamas

Surprisingly, there are some blue holes that also occur on land. Watling's, which is located on San Salvador Island in the Bahamas, is one of those special few who despite sitting in the middle of a rainy tropical island, has no fresh water whatsoever-- not even a tiny lens on top. This all just means that someway somehow, the hole links all the way to the ocean via an underground tunnel that lets salt water flow in to dominate its system. Amazing, isn’t it?

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This article was written by Pamela Chan.