A domed sea cave on the Algarve coast whose vaulted limestone roof is pierced by a great circular skylight — an 'oculus' that pours a column of sunlight onto a hidden beach reachable only from the water.
A sea cave on the tiny island of Biševo that fills with an unearthly blue light around midday, when sunlight enters through a submerged opening and glows up through the water onto the cavern's walls.
A vast limestone cave of seven great chambers lit by natural skylights — and the site where the ancient human species Homo luzonensis was discovered, deep in the karst of the Cagayan Valley.
The largest ice cave on Earth — 42 kilometres of limestone passage in an Alpine cliff, its entrance chambers filled with towering formations of natural ice.
A Bornean rainforest hiding some of the world's greatest caves — including the largest cave chamber on Earth — plus razor-sharp limestone pinnacles rising from the jungle canopy.
A river cave that tunnels for over seven kilometres straight through a limestone mountain — navigated by longtail boat through pitch darkness, from one green valley to another.
The longest cave system known on Earth — over 685 kilometres of mapped passages winding beneath the Kentucky hills, with new miles still being discovered.
Swirling caverns of polished marble at the water's edge of a remote Patagonian lake — their blue-and-white walls glowing turquoise with the reflected light of the glacier-fed water.
An underground lake on the Ionian island of Kefalonia, roofed by a collapsed cavern that lets a shaft of midday sun ignite the brackish water in an unearthly electric blue.
An ancient limestone landscape riddled with the world's greatest cave systems — including Sơn Đoòng, the largest cave passage on Earth, big enough to hold its own jungle, river, and clouds.
A vast karst cave near Kutaisi, its huge chambers hung with stalactites and curtains of stone, explored on lit walkways and, in part, by boat along an underground river.
One of the world's longest navigable underground rivers, winding 8.2 km through a vast limestone cave system beneath a karst mountain range before emptying directly into the West Philippine Sea.
A cave in Romania's Apuseni Mountains that holds one of the largest underground glaciers in the world — a block of ancient ice thousands of years old, deep beneath the forest.
A sea cave on the island of Capri where sunlight passing through an underwater opening fills the whole cavern with an unearthly, glowing electric-blue light.
Thousands of natural sinkholes opening into a hidden world of crystal-clear groundwater and flooded caves beneath the Yucatán — the peninsula's only natural fresh water and among the most beautiful swimming holes on Earth.
