Darkness! Because not even a fraction of sunlight reaches the depths. Since no light, no plants.Pressure! The water exerts tremendous pressure as u go deeper and at great depths of say > 1000 meters, the pressure is crushingCold! Lack of sunlight and the fact that water is a bad conductor of heat means the bottom layers of the ocean are very very coldOccasionally violent! The bottom of the sea represents the thinnest part of the earths crust and hence most prone to earthquakes, volcanoes, tsunamis etc.Rich topography! Mountains, valleys, rivers and beaches … each larger than their counterparts on the surface means a whole undiscovered planetLonely! The cold and dark means that except a few rare organisms like sponges that are specially adapted, there is practically nothing living at the bottom of the sea
2 billion years from now when the sun starts cooling and expanding, it will have engulfed the earth but the heat would have melted the ice on moons like Titan providing mankind wit the next home. However, all such homes are likely to be water planets and how we adapt to living on and below the endless oceans on these planets will determine whether we continue to survive as a species.