Tui De Roy / Volcan Alcedo Giant Tortoise (Chelonoidis nigra vandenburghi) group wallowing in mud at night, Alcedo Volcano, Isabela Island, Galapagos Islands, Ecuador

Nick Upton / Common prawns (Palaemon serratus) and a Rock goby (Gobius paganellus) feeding on a dead Montagu's crab (Xantho hydrophilus) in a rockpool, Cornwall, England, UK, September.

Anup Shah / Western lowland gorilla (Gorilla gorilla gorilla) sub-adult female 'Mosoko' aged 8 years and juvenile male 'Mobangi' aged 5 years drinking from a dirty pool of water in Bai Hokou, Dzanga Sangha Specia...

Mark Hamblin / 2020VISION / Barn Swallow (Hirundo rustica) alighting at pool to collect mud for nest building. Inverness-shire, Scotland, June .Did you know? Due to the distances that Swallows travel, it was traditional for sail...

Anup Shah / Hippopotamus (Hippopotamus amphibius) males fighting in pool, Masai Mara National Reserve, Kenya. February

Sergey Gorshkov / Bubbling geothermal mud pool, Kamchatka, Russia

Anup Shah / Low angle Warthog {Phacochoerus aethiopicus} about to wallow in mud, Serengeti NP, Tanzania

Tui De Roy / Galapagos Giant Tortoise (Chelonoidis nigra) group wallowing in seasonal pool on caldera rim, Alcedo Volcano, Isabella Island, Galapagos Islands, Ecuador

Espen Bergersen / Aerial view of a mud pool in the Gunnuhver geothermal area, Iceland. May.

Ashley Cooper / Fumaroles emitting steam in the geothermal area of Hverir near Myvatn, northern Iceland, September 2010.

Ashley Cooper / Bubbling mud pools in the geothermal area of Hverir near Myvatn, Northern Iceland. September.

Will Watson / Anoxic mud with Cynobacteria and rivulets of black anoxic bacteria on the edge of Upton Warren saline flash pool, Worcestershire, England, UK, August.