Juan Manuel Garcia-Ruiz, Spanish geologist. He is holding a sample of the selenite crystals found in the Naica Mine, Chihuahua, Mexico. This mine contains caves in which are found the largest natural crystals in the world, over 11 metres in length in some cases. Garcia-Ruiz worked out how such large crystals formed. The caves were filled with geothermally heated water, which was at the critical temperature for the formation of crystals. Calcium sulphate was constantly replenished by water percolating through the rock. Extremely gradual cooling in these conditions allowed the huge crystals to form. The caves were discovered when the water was pumped out of the mine. Photographed in 2006.