Vincent Grafhorst / Water dripping from stalactites creating stalagmites below, Gcwihaba Caves, Botswana

Vincent Grafhorst / Cave entrance, Gcwihaba Caves, Botswana

Piotr Naskrecki / Tailless Whip Scorpion (Daemon variegatus) in cave, Gorongosa National Park, Mozambique

Doug Wechsler / Eastern Subterranean Termite (Reticulitermes flavipes) worker, with larvae and eggs in rotten log, French Creek State Park, Philadelphia, Pennsylvania, USA, August.

Doug Wechsler / Eastern Subterranean Termite (Reticulitermes flavipes) worker, with larvae and eggs in rotten log, French Creek State Park, Philadelphia, Pennsylvania, USA, August.

D. Parer & E. Parer-Cook / Short-beaked Echidna (Tachyglossus aculeatus) juvenile called a puggle in its burrow, Australia

D. Parer & E. Parer-Cook / Short-beaked Echidna (Tachyglossus aculeatus) juvenile called a puggle in its burrow, Australia

Cyril Ruoso / European Rabbit (Oryctolagus cuniculus) in underground burrow, France, introduced worldwide

D. Parer & E. Parer-Cook / Galapagos Land Iguana (Conolophus subcristatus) group keeping warm in hot caves, Fernandina Island, Galapagos Islands, Ecuador

D. Parer & E. Parer-Cook / Water-holding Frog (Cyclorana platycephala) breaking from skin after rain, Australia

D. Parer & E. Parer-Cook / Water-holding Frog (Cyclorana platycephala) breaking from skin after rain, Australia

D. Parer & E. Parer-Cook / Water-holding Frog (Cyclorana platycephala) underground in skin before rain, Australia