10 Wondrous Water Caves

A cave entrance and natural arch along the coastline
A natural arch on the coast of Capri, Italy marks the entrance to a narrow sea cave called the Blue Grotto.

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Many unique caves in the world are found on the shores of oceans and lakes. Some of these water caves are expansive systems that can extend under the ocean for long distances and are only accessible to divers. Others, found along rocky coastal cliffs, can be explored via canoe or other small boats. Some, carved by flowing rivers, are easiest to access on foot.

While many water caves have been created by erosion from pounding waves or flowing waters, others are the result of changing sea levels during glacial periods, also known as ice ages. These caves, which once stood above ground, were flooded by rising seawater thousands of years ago. No matter how they were formed, water caves are natural spectacles of geology that attract visitors the world over.

Here are 10 amazing water caves found across the planet. 

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Fingal's Cave

Ocean water enters a cave in a rocky cliff with a columnar appearance

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Located on the island of Staffa, Scotland, Fingal's Cave is known for its natural acoustics and unique geology. The cave is said to produce harmonic echoes and is the source of inspiration for Felix Mendelssohn's Hebrides Overture. The cave, along with the entire island of Staffa, is composed of hexagonal basalt columns (similar to those of Giant's Causeway) as a result of volcanic activity. 

Due to the cave's coastal position, its bottom is filled with seawater. Measurements from different sources vary, but the cave is thought to be about 70 feet tall from the water's surface, with another 100-150 feet stretching below water.  

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Sea Lion Caves

Sea lions lie on rocks near the mouth of a sea cave

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The Sea Lion Caves, found on the Oregon Coast, is the only mainland breeding ground of the Steller sea lion, a near-threatened species. It's also the longest sea cave in North America, measuring 1,315 feet long. Most of the cave is at sea level and is flooded with water at high tide. One section of the cave, however, sits 50 feet higher and serves as a platform to view the sea lion colony, as well as colorful lichens, algae, and minerals that cover the cave walls. 

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Alofaaga Blowholes

Water is expelled into the air through holes in a stretch of rocky coastline

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The Alofaaga Blowholes are a series of natural tubes that connect the ocean to the rocky coastline on the island nation of Samoa. When high tide arrives, breaking waves are jettisoned up through the tubes, creating high-pressure water geysers that erupt from the ground with every wave. Due to the force of the water, getting too close to the blowholes could be dangerous, and falling in one of the tubes would likely prove deadly.

The openings, which are a geological feature known as lava tubes, are the product of volcanic activity. Lava tubes are formed when lava flows and cools at different rates. Cooling lava hardens and becomes solid, while hotter lava below it continues to flow like a liquid, creating a tube.

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Great Blue Hole

A circle of darker blue water in the ocean reveals an underwater cave, surrounded by white reefs

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The Great Blue Hole, located in the Lighthouse Reef off the coast of Belize, is a vertical cave that's entirely underwater. It measures 984 feet in diameter and 410 feet deep. With its beautiful, clear water and a variety of wild marine life residing in its depths, the Great Blue Hole is most famous as a scuba diving destination.

Although the Great Blue Hole is one of the most spectacular, it's not the world's only blue hole. The term is used to describe any submarine, vertical cave, which are found in oceans across the planet. Blue holes formed during past ice ages when sea levels could be as much as 300 feet lower than they are currently. In most cases, blue holes were carved out of soft limestone landscapes while still above ground and then flooded as sea levels rose. 

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Blue Grotto

People paddle boats through a water cave, illuminated by sunlight

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Known for its gorgeous blue water, Italy's Blue Grotto is one of the most famous sea caves in the world. Though the water here is exceptionally clear, the lighting of the cave is also responsible for its stunning blue aura. The cave has two openings: a small, narrow one above the water surface, and a larger one underwater. The larger opening is responsible for most of the sunlight entering the cave. The sunlight disperses as it passes through the water into the cave, making it seem as though the water itself is a source of light.

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Painted Cave

A cave entrance with bumpy rock walls and a floor covered by bright blue water

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The Painted Cave, located on California's Santa Cruz Island, is an expansive sea cave known for the colorful rocks, lichen, and algae found on its walls. After the Sea Lion Caves, it's the second-longest sea cave in North America, at 1,227 feet long.

The cave is a popular kayaking destination since the cave's bottom is underwater for its entire length. Deep in the cave, the passageway becomes narrower before spilling out into a cavernous chamber, which is nearly pitch black and often occupied by sea lions. 

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Apostle Islands Sea Caves

Two people stand at the entrance of a cave on the edge of a frozen lake

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Wisconsin's Apostle Islands Sea Caves are sandstone caves on the shores of Lake Superior that feature brilliant icicle displays in winter. Devils Island, Sand Island, and a stretch of mainland Wisconsin all have caves, which together are known as the Apostle Islands Sea Caves. 

The caves are accessible by boat in summer, but seeing the caves during winter has become a popular, if weather-dependent, alternative. When Lake Superior freezes, the ice is sometimes thick enough to allow hikers to walk across the lake and explore the caves. But Lake Superior never freezes entirely, and strong winds can whip up waves that destroy the ice, ruining the passageway until it freezes once again.

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Smoo Cave

A walkway and bridge lead into a large cavern

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Smoo Cave is a limestone cave in Scotland that has been carved by both seawater and a freshwater river. The mouth of the cave, formed by seawater erosion, is evidence of a time when sea levels were higher. Today, seawater only barely reaches the cave during king tides—the strongest tides, which occur during full and new moons. 

The smaller chambers, found deeper in the cave, have been carved by Allt Smoo (Gaelic for Smoo River). The river enters the cave from above, cascading into the cave through a sinkhole. 

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Cuevas de Mármol

A series of caves in a slab of smooth rock on the coastline

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The Marble Caves, or Cuevas de Mármol, are carved into marble blocks on the shores of Patagonia's General Carrera Lake, which straddles the border of Chile and Argentina. The smooth marble walls reflect the glacial lake's striking blue-green water, creating a unique ambiance. 

Eroded by the lake's waves over thousands of years, the caves are a complex network of tunnels and passageways situated just above the water's surface. They are a popular tourist destination and can be seen by paddling a kayak across the lake. 

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Hidden Beach

A sandy sea cave with a circular opening and a view of blue sky

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Hidden Beach is a secluded, sandy cave on the Marietas Islands, a group of uninhabited islands in central Mexico's Banderas Bay. The cave, also known as Playa del Amor, is essentially a hole in the ground with steep overhanging walls that reveal a picturesque beach and sea cave. The beach is connected to the Pacific Ocean by a narrow tunnel, which is inundated with water at high tide, but offers visitors access to the beach at low tide. 

The islands were designated as a national park in 2004, and since then Hidden Beach has become a prized tourist destination. In 2016, the islands were closed to tourism due to concern that the number of visitors was damaging the islands' ecosystem. The islands and Hidden Beach have since reopened, with regulations to limit the number of visitors per day.