Alex Hyde / Green Sea Urchin (Psammechinus miliaris) in a rockpool, Isle of Mull, Scotland. June 2010.

Juergen Freund / Snorkeler in hidden underwater cave of Nuapapu, Tonga, Melanesia, 2007 .

Juergen Freund / Snorkeler near limestone caves, Vava'u Islands, Tonga, Melanesia, 2007

Juergen Freund / Limestone caves, Vava'u Islands, Tonga, Melanesia, 2007

Juergen Freund / Whitetip reef shark {Triaenodon obesus} in cave, Gorgona Island, Colombia, Pacific ocean.

Juergen Freund / Sponge {Axinella verrucosa} in underwater cave, Mediterranean