This is the world's largest collection of open-air Palaeolithic art (10,000 to 40,000-years-old). And to think it all nearly disappeared under water.

This stretch of valley peppered with 17 kilometres-worth of rock engravings was only rediscovered in the late '80s when government surveyors began scoping out the area for a potential hydroelectric dam. When salivating archaeologists got wind of the Stone Age discovery, a fierce battle ensued to save the valley.

The rock shelter etchings - including thousands of images of horses, ibexes, red deer, fish, aurochs (ancient wild cattle) as well as human figures - changed the way archaeologists view Stone Age art. In 1998, UNESCO declared it a World Heritage Site.

The Côa Valley is situated in Portugal's northeast, only 15km from the Spanish border. The closest town is Vila Nova de Foz Coa.