The deep sea is a very dark and cold place. The deep sea goes all the way down to 11,000m (so it is deeper than Everest is tall!) and there is no sun light below 200m. There is also very little food and the weight of the water creates an incredible crushing pressure. Unfortunately for us this pressure is so great we would not survive in the deep sea but lucky for the animals living there they have some pretty clever strategies that do allow them to survive!
For a long time lots of scientists believed there was little to no life in the deep sea but we have discovered a huge number of species even just in the past 60-70 years. And we are discovering more species with every sampling survey we do!