Why there are more exploration on moon than on ocean floor ?

The pressure of traveling to the ocean floor is so strong that can crush things so easily. If a space shuttle had a crack in it then it's probably trapped, but you could send a new one up to try and retrieve the people. It would all depend on WHEN and what happened. Now if something were to happen where there was a crack in a sub if someone was in it they would be killed basically instantly. The pressure would be so great that they couldn't survive. That risk factor IS an issue, just rarely mentioned.