Gerrit Vyn / Jonah natural gas field south of Pinedale, Wyoming.

Alex Hyde / Chlorophyte or green alga (Volvox aureus) in pond water. UK. The larger spheres are approximately 0.5mm in diameter. Digital composite image.

Pal Hermansen / Composite of flying butterflies (Lepidoptera) on sphere.

Paul Williams / View from plane of eroded salt dome, Kavir Desert, Iran, December.

David Fleetham / Trumpetfish (Aulostomus maculatus) swimming behind Yellow boxfish (Ostracion cubicus) as a blind to ambush prey. Yap, Micronesia.

David Fleetham / Trumpetfish (Aulostomus chinensis) and Longjaw squirrelfish (Sargocentron spiniferum) with school of Bigscale soldierfish (Myripristis berndti) behind. Coral reef, Maui, Hawaii.

David Fleetham / Spotted boxfish (Ostracion meleagris) swimming above sea floor. Hawaii.

Graham Eaton / An updraft (vortex) of spiralling air and moisture caused by air being displaced by falling water. Pistyll Rhaeadr waterfall, located a few miles from the village of Llanrhaeadr-ym-Mochnant in Powys,...

Martin Gabriel / Rounded boulder with crack, Erebuni State Reserve near Yerevan, Armenia

Pal Hermansen / Spherical stone on Champ Island, Franz Josef Land, Russia, July 2004.

Franco Banfi / Blue jack mackerel, (Trachurus picturatus) baitball, Formigas Islet dive site, Azores, Portugal, Atlantic Ocean

Juan Carlos Munoz / Boruca woman with stone spheres, created by the ancestors of indigenous Boruca peopla, Costa Rica. December 2014.