Blue holes are among the world’s most dramatic natural formations, offering an astounding look into our planet and boasting eye-popping shades of blue seen nowhere else on earth. Exploration of these sunken realms reveals spectacular scenery, fascinating archaeological finds, and mysterious caverns and caves previously believed to be entries to the underworld - not gateways to adventure. These otherworldly dive sites are found in karst landscapes all around the globe, and they come in a wide variety of shapes, sizes, and depths. So, there’s an option for exploration no matter what skill or certification level you’re diving at.
Stay tuned for all you need to know before taking the plunge in these fascinating geological features found all around the world.
What are blue holes, and how did they form?
Blue holes are actually flooded sinkholes - naturally occurring depressions formed by collapsed limestone and bedrock. These marine caverns are open to the surface, and often feature sheer vertical walls, white sand or silt bottoms, and profound depths, giving them a stunning azure hue.
These fascinating formations are typically found in karst stone landscapes, and are often part of larger cavern and cave systems connected to the surrounding ocean. So, they often boast a blend of fresh and saltwater, as well as interesting submerged features including stalagmites and stalactites.
Most blue holes developed during the last ice age. As the planet’s polar ice caps froze and sea levels dropped, limestone rocks and coral reefs were exposed to the elements, allowing erosion to wear away at them and create areas of collapse. Then, at the end of the ice age when the polar ice caps melted again and sea levels rose, the collapsed areas or sinkholes flooded. This is why blue holes often contain fascinating fossils and artefacts dating back thousands - or even millions of years. Often left undisturbed for millennia, these underwater treasure troves now lend valuable insight into how our world’s oceans have evolved and changed.
Our planet has changed dramatically since the last ice age, and so have its blue holes. In some areas like Australia’s Great Barrier Reef and Egypt’s Red Sea, vast coral gardens have colonised the area, growing right up to the edges of these sinkholes. In others, such as Mexico’s Yucatan Peninsula, jungles and rainforests now dominate the landscape giving an otherworldly feel to the formations. It’s no wonder the ancient people who settled there believed them to be the gateway to the underworld!
What to expect from blue hole diving
Because blue holes are protected from the currents and waves found in the open ocean, they often host their own unique ecosystems. And you might encounter a variety of fascinating and even prehistoric finds while exploring them! Fossils, ancient coral formations, cave and cavern-like features, and haloclines - a visual effect where fresh and salt waters meet - can all be observed within their walls. You may also spot strange endemic species of cave fish, shrimp, and other crustaceans.
It’s important to keep in mind that blue holes look very different from above than at sea level. When diving these geological formations, you likely won’t be able to see the contrast in water depth and clarity - the famous shade of deep blue is less obvious inside the blue hole. And because their walls are often made of sheer limestone, you can expect an experience more similar to diving in caverns, caves, or cenotes than the open sea.
Some of the most fascinating features found in blue holes are located at depth, making them a huge draw for tech divers. The passageways to enter and exit these sites may include “full overhead” environments - or swim-throughs that don’t offer direct access to the surface, so they’re best suited to advanced underwater explorers with outstanding buoyancy control and experience diving to 30 metres or below.
If you’re planning on giving cave or cavern diving a try or exploring beyond the recreational diving limits, it’s essential to carry the proper certifications. You should also take the time to practice your new skills before going on holiday, this will boost your confidence, help you dive safely, and allow you to relax and enjoy every tank while you’re away!
Ready to start exploring Mexico's cenotes?
Search, compare and book from our selection of resorts in Yucatan
Where to dive blue holes
The rock formations where blue holes typically form can occur in a wide variety of environments all around the world. But, the most stunning are found in tropical waters close to the equator.
The Great Blue Hole of Belize
Originally discovered by Jacques Cousteau and now a UNESCO World Heritage Site, this is easily the planet’s most widely known blue hole. So, it’s no surprise that Great Blue Hole diving is among Belize’s top tourist attractions, despite the incredible marine life found in other parts of the country. This sinkhole lies near the centre of Lighthouse Reef, a pristine atoll found off the Caribbean coast of Belize. Most visits include two to three tanks, with one dive exploring inside the blue hole, and the rest of the day’s diving focused on its stunning surroundings.
While inside the Great Blue Hole, you’re likely to spot a range of marine life, including midnight parrotfish, reef and nurse sharks, and a wide variety of juvenile reef fish. And, unlike many other blue holes, this natural formation has nearly vertical walls and very little horizontal development - so you won’t find many cavern or cave-like passageways to explore. Experiencing the Great Blue Hole of Belize’s diving generally requires an entire day on the water, with most trips lasting eight hours or more.
Egypt’s Blue Hole
Just a few kilometres north of Dahab, this site, often simply referred to as The Blue Hole, is one of Egypt’s most famous diving sites. Unlike some of the other sites on this list, this blue hole is attached to a cave system, often explored by divers. Two diveable passageways lead in and out of the formation, known as “The Saddle” and “The Arch”. The Saddle is a shallow channel, found at around six metres, whereas The Arch begins well below the recreational diving limits, at around 55 metres. A second dive site, known as “The Bells”, is also combined with exploration of the blue hole, allowing divers to enter from shore, explore a plunging wall, and then enter the blue hole through “The Saddle”.
The Blue Hole is one of Egypt’s most sought-after sites for recreational divers, but it’s also a favourite among more adventurous underwater explorers. Because much of this site’s features begin below 30 metres, it’s become a hotspot for technical divers - with numerous dive centres in Dahab offering courses on doubles, sidemount, trimix, and more. Freedivers flock here because of the site’s incredible vertical descent, easy access and profound depths, just a few fin kicks from the shore.
The Bahamas’ Blue Holes National Park
The Bahamas are actually home to hundreds of blue holes, but just 50 of them have been explored by divers, and a mere seven are open to recreational exploration. Found on and around the Bahamanian island of Andros, the Blue Holes National Park covers a whopping 40,000 acres and is home to the most famous and impressive of these underwater formations. Andros blue hole diving is different from other sinkhole exploration, as many of the sites are predominantly freshwater. This means their underwater formations remain intact, and helps to preserve prehistoric finds including a variety of fossils!
The Lost Ocean Blue Hole is among the most famous underwater sinkholes in the Bahamas and one of the most accessible, open for exploration by divers of all levels, including total beginners. Reef sharks, stingrays, and massive groupers can all be found within its walls. Other blue holes found within the park, like Stargate, require technical certification, as their passageways are long and potentially perilous, with unusual underwater conditions. Rare endemic species, ancient human remains, and fascinating relics from the past can be found within these more complex cave systems.
Malta’s Blue Hole in Gozo
This site is Malta's most popular shore dive, and is surrounded by stunning rock features recently featured in the television series, Game of Thrones. The underwater portion of this limestone formation is similar to Dahab’s blue hole, as it connects to the sea via an underwater arch, found at a depth of 15 metres. This gateway leads to lovely reefs on the outer side of the wall, ideal for easy-going exploration either before or after the main attraction.
Malta’s Blue Hole attracts divers with its spectacular underwater rock formations and incredibly atmospheric lighting at depth. And, because the sinkhole is entirely encircled by rocks, it packs a completely different visual impact than its tropical counterparts. This is one of the smallest and shallowest blue holes on our list as well, making it ideal for less experienced explorers and even snorkellers! You can access the site from land or sea, with a wide variety of options for entrance and exit, based on personal preference.
Guam’s Blue Hole
Found off the west coast of Guam, this is one of the island’s most requested dive sites - even though it doesn’t look like much from the surface. This blue hole is entirely submerged, so it doesn’t have the signature appearance of a deep pool of blue. But, it’s a fascinating site, offering a wide variety of options for exploration and crystal clear water, with visibility topping out at over 30 metres all year round. Because its entrances and exits are submerged, this blue hole’s walls are covered in soft and hard corals, and it attracts a wide variety of reef fish and critters taking shelter from the open ocean.
The heart-shaped entrance to this sinkhole lies at about 18 metres, allowing for easy access. Once inside, divers can choose their own depth, either returning to the entrance and surfacing or exiting through a window at around 40 metres. Just outside this deep-water channel, divers will encounter a plummeting wall, often patrolled by sharks and other passing pelagics. This is an easy site to visit, with a 20-minute boat ride from shore, and exploration here is usually combined with at least one more dive, highlighting the region’s thriving coral reef ecosystem.
Swain Reefs National Park, Eastern Australia
Located around 200 kilometres off the coast of Yeppoon, this is one of The Great Barrier Reef’s least dived areas - simply because it is so remote. Swain Reefs, locally referred to as just Swains, is only dived via charter, and encompasses nearly 300 individual reef systems. So, it’s no wonder that the region’s blue holes went undiscovered for so long. In fact, the most famous of them, the Crystal Blue Hole, was only discovered when researchers spotted something on a Google Earth satellite image, and was later explored by local dive guides.
Surrounded by a shallow turquoise lagoon, the Crystal Blue Hole offers an easy and protected anchorage, and stunning dive sites boasting dramatic sand channels and coral bommies all around. Once inside, the walls drop nearly-vertical to 45 metres, with stunning limestone formations and lovely gardens of hard and soft coral all the way down. Because the current and temperature remain constant, the formation’s interior has been protected against the elements, allowing its internal coral structures to remain pristine. This is especially important, as the area is prone to seasonal coral bleaching and tropical cyclones! Because this blue hole was so recently discovered, little is known about what wildlife might be spotted inside, making it one of the most exciting frontier diving destinations on this list!
Are you ready to start planning the deep blue hole dive of your dreams?
ZuBlu has your back! Our team of travel experts have all the latest info, no matter where in the world you’re hoping to explore. And, we’re happy to guide you through the entire experience from start to finish for a perfectly seamless dive holiday.