Colin Varndell / Great spotted woodpecker (Dendrocopos major) female drumming on tree branch in winter, Dorset, UK January

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.

Juergen Freund / Cultured Golden South Sea pearls of the Pearl oyster (Pinctada maxima) Jewelmer pearls, Philippines

Jack Dykinga / Concretion balls lining fracture lines in petrified sand dune formation, with lichen scaring calcification marks, Colorado Plateau, Arizona, USA

Nick Upton / Common pond skater (Gerris lacustris) standing on the surface of a garden pond before skating off at speed, with many colonial Green algae (Volvox) spheres moving below the surface, Wiltshire, England...

David Fleetham / Sculptural concrete pillars holding polished steel spheres with 610m Guangzhou TV and Sightseeing Tower visible across Zhujiang River. Guangdong, China. For Editorial use only.

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.

Bryan and Cherry Alexander / Self portrait of photographer Cherry Alexander in the reflection of the metallic sphere at the ceremonial South Pole, Antarctica.

Bryan and Cherry Alexander / Photographer Cherry Alexander at the ceremonial South Pole, Antarctica.