A blue hole, an arch, a pinnacle, a crack, a lake and more that will blow your mind ... here are the natural wonders of the underwater world that are prime for scuba diving.

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Silfra Fissure | Iceland

To scuba dive between the Eurasian and North American continental plates, start from the shore of Iceland’s Thingvellir National Park, 35 minutes east of Reykjavik by car. Follow the stairs to the freshwater lake, so clear that swimming in it comes with its own set of bragging rights.

Says Hossi Elefsen, manager of Dive.Is, “The Big Crack, as we call it, is filled with glacial water that’s been filtered through porous underground lava rock for 30 to 50 years. This is what allows for visibility of 100 feet and beyond — it doesn’t get any clearer than this.”

But the clear water alone isn’t the attraction worth the travel. Like the Four Corners Monument in the southwestern U.S. that lets visitors touch four states simultaneously, the Silfra Fissure is fun in that the walls of the two plates are so close that divers can place one hand on each. Says Elefsen, “There really is nowhere else in the world like it.” dive.is

Great Blue Hole | Belize

A trip to Belize’s Great Blue Hole, roughly 60 miles off the coast, isn’t about the wildlife that the sinkhole supports, but rather the experience of witnessing the stalactites and other geological features dating back some 150,000 years. It’s hard to imagine, but the last time glaciers crept across this now jungle, the Great Blue Hole was a cave. Slowly, a rising sea level caused the ceiling to fall in. The result is this shaft, whose bottom — at 394 feet — was at sea level 18,000 years ago.

And while seeing lobsters and sharks might be possible, what are certain are the fat knobs of stalactites, larger around than most divers. Together, they create a garden of columns, which, for some to encounter, is a history-making experience all its own. aggressor.com

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Tongue of the Ocean | Bahamas

This stocking-shaped oceanic trench cuts between the islands of Andros and New Providence. It starts in 45 feet of water and bottoms out at 6,500 feet. Says Hartman Rolle, operations manager of Stuart Cove’s Dive Bahamas: “It gives you that eerie feeling of not seeing the bottom. This channel sees a lot of migrations from the Florida Keys, including sperm whales, sharks, lots of ocean fish — you name it.” stuartcove.com

Darwin’s Arch | Ecuador

Impressive as it is above the surface, Darwin’s Arch — a natural stone bridge off the northernmost of Ecuador’s Galapagos Islands — will have your jaw dropping as soon as you descend. That’s because the formation sits atop a plateau called the Theater. It’s there that schools of jacks, scalloped hammerheads, rays and bonito — numbering in the hundreds and even thousands — appear. The island of Darwin is the tip of an extinct volcano sitting amid the open ocean, with nothing else, save for neighboring Wolf Island, in sight for 115 miles. It’s as close as divers can get to experiencing nature’s traffic jams powering along oceanic currents. aggressor.com

Lake Baikal | Russia

More than a mile deep, Russia’s Lake Baikal, located north of central Mongolia, is an inland wonder and home to freshwater species found nowhere else. Start with the Baikal seal — the world’s only pinniped that does not rely on salt water. Gammarus, amphipod crustaceans, have thrived in evolution there, growing to the size of a fist, whereas elsewhere in the world, they’re no bigger than a pinky nail.

Beyond the life it supports, the lake’s underwater topography stuns most visitors, especially those who assume wall diving is limited to the ocean. A pulling apart of Earth’s crust created the lake, giving it sheer walls slicing directly to the bottom at 5,387 feet.

Baikal — the world’s largest freshwater lake by volume — contains a whopping 20 percent of Earth’s unfrozen fresh water. waterworld.at

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Hart Springs | Florida

Don’t be surprised if Hart Springs is among Florida’s caves you haven’t heard of yet. To enter this site outside the town of Bell, two hours west of Jacksonville, you need to present a Full Cave Diver certification, as well as what’s called the Abe Davis Award, aka proof of completing 100 safe cave dives. Then there’s the flooding, which happens so often that the cave is closed for months at a time.

But when conditions and training align, this is, according to Florida cave diving expert Jill Heinerth, one of the most pristine systems in the state.

“On the surface, there are these giant water cabbage lettuces, and it doesn’t look like the kind of place you’d want to dive, but inside it’s just gorgeous.”

Plus, within its walls lie the remains of an ancient alligator, other Paleozoic Era animal remains and pottery. For those who lack the experience to dip inside Hart Springs, the trip can still be worth it. Says Heinerth, “Hart creates a spring that runs into the Suwannee River, and snorkeling that spring run is gorgeous in and of itself.” amigosdivecenter.com

Great Barrier Reef | Australia

The world’s largest coral-reef ecosystem suffered from bleaching in 2016, but with more than 2,900 separate coral reefs spanning a range of 14 latitudinal degrees, the majority of biodiversity there remains intact.

Consider that these islands are home to 1,625 species of fish, including bumphead parrotfish, potato cod and barramundi cod. Rare but not impossible to see are whale sharks, green sawfish and dugongs.

But what’s perhaps most impressive is the sheer number of marine life in this blue wilderness. At Cod Hole, 12 or so potato cod show for every dive. At North Horn, it’s common to see 20 to 30 gray reef sharks on a single dip. Species here thrive not in isolation, but rather in populations so large that these reefs feel like another Wild West. mikeball.com

Molokini Crater | Hawaii

Found 2.5 miles off Maui’s south coast, Molokini Crater is the type of wall that divers dream about. For starters, the crescent-shaped volcanic formation has no soil, and thus no run-off, allowing for visibility far in excess of 100 feet. Its backside is sheer, falling 160 feet straight down in the Alalakeiki Channel. It’s nearly a given that you’ll encounter whitetip sharks, schools of amberjack, and green sea turtles pausing for a cleaning from Hawaiian cleaner wrasse. Add a bit of luck, and sightings of mantas, blacktips and even whale sharks are possible. mauidiveshop.com

Eye of the Needle | Saba

Timing is everything when dropping onto Eye of the Needle. Strong currents push past this pinnacle a mile off the shore of Saba, a satellite of St. Maarten. To reach it, divers must free-fall, quickly reaching its top 90 feet from the surface. From there, this tower of coral drops to 225 feet. “Even on an average day of visibility, you can see all the way to the bottom,” says Caroline Levy, head instructor at scuba center Sea Saba. seasaba.com