We follow Alberto Angela into the depths of the abyss. The world of the deep sea is still little known and full of surprises. We know the surface of the Moon in more detail than the depths of our seas. In this episode we discover the stories and adventures of the pioneers who led humanity to discover the oceans and their underwater world. "Challenge to the abyss" takes us on board one of the most advanced submarines of the Italian Navy, the Todaro, to discover what happens on board a modern submarine and how its crew lives.