Venture deep beneath Mexico to visit one of nature's greatest wonders - a shimmering cathedral packed with 11-metre-long crystals, 1000 feet down in a working lead mine. This secret place is as deadly as it is beautiful. With a temperature of 45 degrees Celsius and humidity over 90 per cent, it could kill an unprotected human in 30 minutes. In fact, on the first scientific mission to explore it in 2007, it very nearly did. Despite the danger, our team has returned to revisit one of the world's most amazing subterranean spaces. The caves are gradually being re-submerged in the scalding waters that formed the crystals. In a race against time, one team explores the original site while another group risks their lives by descending into a newly drilled shaft in the hope of discovering an interconnected system of underground chambers.