Digging and excavating are bywords for archaeology. But why does history end up deep under our feet? This question struck CrowdScience listener Sunil in an underground car park. Archaeological remains found during the car park’s construction were displayed in the subterranean stairwells, getting progressively older the deeper he went. How had these treasures become covered in so much soil over the centuries? CrowdScience visits Lisbon, the capital of Portugal – and home to the above-mentioned multi-storey car park. The city has evidence of human habitation stretching back into prehistory, with remnants of successive civilisations embedded and jumbled up below today’s street level. Why did it all end up like this? Human behaviour is one factor, but natural processes are at work too. Over at Butser Ancient Farm, an experimental archaeology site in the UK, we explore the myriad forces of nature that cover up – or expose - ancient buildings and artefacts over time. Contributors: Dr Mariana Nabais, University of Lisbon Carolina Grilo, Lisbon Museum of the Roman Theatre Dr Matt Pope, University College London Presented by Marnie Chesterton, Produced by Cathy Edwards for the BBC World Service. IMAGE: Getty Images