The gigantic Gruta de Benagil sea cave (Benagil Cathedral), Algarve, Portugal

Environmental information poster about ecosystem of limestone cenotes, Yucatan, Mexico, North America

Looking down into the Los Peces fresh water cave near Playa Giron, Matanzas Province, Cuba, West Indies, Caribbean, Central America

School of silversides form shapes in the tunnels at Eden Rock, Grand Cayman.

A scuba diver swims through an underwater field of tape grass leading away from the Blue Springs cave in Merritts Mill Pond in Marianna, Florida.

An adult American Eel (Anguilla rostrata) prowls along the edge of the Morrison Springs cavern at 85 feet deep watching over the adolescent eels in the clear freshwaters of the state park near Red Bay, Florida.

Waterfall from Slate Cavern Tunnel entrance, Dinorwig Quarry (Dinorwic Quarry), Eryri (Snowdonia National Park), North Wales, United Kingdom

Vrelo Cave near Canyon Matka, Macedonia, Europe

Porous sand stone rocks on Jalandhar Beach near the south pole point in Diu, India, Asia

Castellana caves (Castellana Grotte), Apulia, Italy, Europe

Microbats constitute the suborder Microchiroptera, found in a sandstone crevice on Bigge Island, Kimberley, Western Australia, Australia, Pacific

Split image of lush vegetation and free diver observing lettuce coral, Turbinaria reniformis, next to cave entrance, Palau, Micronesia.

Whitetip reef shark, Triaenodon obesus, resting in cave, Namu atoll, Marshall Islands (N. Pacific).

Dwarf Mongoose

People kayaking in Opara Basin, Box Canyon, West Coast, South Island, New Zealand, Oceania

Crinoids and a small gorgonian fan, Raja Ampat Indonesia

Woolly horseshoe in a cave, Bandhavgarh NP India

Woolly horseshoe in a cave, Bandhavgarh NP India