A mysterious group of rusting ships stranded in an arid desert; a monstrous defensive line on the snow-covered peaks of the Italian Alps; a ghostly collection of ornate buildings deep in the German woodland; the sunken remains of a unique maritime vessel with a story to tell... Once these extraordinary places and objects were at the cutting edge of design and construction, but now they stand disused, sometimes contaminated and sometimes dangerous. But who was it that built these mysterious structures? What secrets do they hold, and why were they abandoned?

In this episode, concrete channels hidden in a mountaintop forest, a collapsing bridge, a marooned military base and the upright rusting carcass of ship - but why have they all been abandoned?