Incredible feats of human endeavour, revealing what people have built, where they've built it, how and why.
Episode 2: Sweden
A look at Sweden's passion for subterranean spaces and how this nation has taken extraordinary precautions to protect itself - by excavating an astounding 65,000 nuclear bunkers. In south-west England, deep in the Mendip Hills of Somerset, lies one of the most spectacular landscapes in the UK, but beneath this breath-taking landscape sits secret caverns and magical rock formations carved out by nature. Plus, how in Romania, nside a 1.2km-thick layer of salt, sits a world underground first.