Subterranea peels back the outer layer of the earth and reveals the fascinating hidden underground spaces that you cannot see from above the ground. These places include the poisonous caves in Mexico, full of deadly hydrogen sulphide, where the toughest fish in the world manage to survive; the magnificent Roman sunken palace that was lost beneath the streets of Constantinople for centuries; the 'Door to Hell' that was accidentally created by Soviet gas explorers in the 1970s and has been on fire for nearly half a century; and the drug-smuggling tunnels between Mexico and the USA.