Jen Guyton / Lump of hardened earth criss-crossed with tiny tunnels showing the presence of a termite nest inside a fairy circle in the Namib Desert, Namibia. February 2015

Chris Mattison / The Headstone Tunnel, 490m long, formerly part of the Buxton - Matlock railway line, now part of the Monsal Trail cycle route. Opened 2011. Peak District National Park, Derbyshire, UK

Franco Banfi / Scuba diver exploring Little Crack, almost like a miniature version of the main fissure, with both deep and shallow sections and many tunnels, Silfra Lagoon, Thingvellir National Park, Iceland.

Nick Upton / Eroded tunnel through a large volcanic rock, named 'Kalikatsou', above Petra beach, Patmos, Dodecanese Islands, Greece, August 2013.

Graham Eaton / An adit in an abandoned slate mine (Cambrian age) at Tanygrisiau, Wales February 2014

Franco Banfi / Tourist walking through man-made tunnel through lava flow, Galapagos Islands, East Pacific Ocean

Alex Mustard / Diver swimming through Tolberone Tunnel, a triangular tunnel through the middle of an island in Raja Ampat. Giant barrel sponges (Xestospongia muta) growing on the floor of the tunnel, which have an u...

Juan Carlos Munoz / Mengore Hill Museum, the cleaned and restored remains of the first Austro-Hungarian line of defense from the First World War,. Walk of Peace, Soca Valley, Julian Alps, Slovenia, October 2014.

Enrique Lopez-Tapia / Stairs in an access tunnel to the churches of Lalibela. UNESCO World Heritage Site. Ethiopia, December 2014.

Enrique Lopez-Tapia / Priest walking through tunnel to Bet Maryam (part of the northwestern group of churches in Lalibela). UNESCO World Heritage Site. Lalibela. Ethiopia, December 2014.

Enrique Lopez-Tapia / Priest walking through tunnel to Bet Maryam (part of the northwestern group of churches in Lalibela). UNESCO World Heritage Site. Lalibela. Ethiopia, December 2014.

Paul Williams / Goshawk (Accipiter gentilis) flying through narrow tunnel in experiment. Taken for BBC series 'The Animal's Guide to Britain'.