Overcoming the forces and pressures of deep water has been more of a challenge than getting to space. It's so dark and so hard to get a clear picture that we don't know what to look for or where to start for one. The task of overcoming the pressure from the weight of all that water combined with it being a constantly moving fluid is incredible.
Traditionally we focused on what was close to shore or close to the surface. As time passed and our knowledge and technology grew, we were able to go further and deeper. Perhaps, eventually, we'll know it all. The difficulties Glis raised are very valid.