In major cities around the world today, skyscrapers loom overhead, taxis honk their horns and street vendors peddle their wares. But below lies city upon city - each with its fascinating and unknown history. This series will explore these layers, which are often hundreds of feet deep, and examine a city's ancient cisterns, dank dungeons, eerie tombs, clandestine hideouts, and even underground shipwrecks that have been lost for hundreds and hundreds of years. What are the underground layers of the most populated cities on earth? And how did (and do) engineers build layer upon layer up to today's city streets? This series will reveal the technological marvels that allowed the construction of one city upon another - literally.