The Soviet Union built countless engineering marvels beyond the Iron Curtain, but today, these mysterious places are abandoned; experts use cutting-edge tech to reveal the secrets of how they were built and why they were abandoned. The segments of this special come from various episodes.

Many of the world's most mysterious abandoned buildings are hidden underwater and underground. But now, experts use cutting-edge tech to reveal the secrets of these strange places and finally explain why they were deserted. The segments of this special come from various episodes.