It’s REALLY heavy down there!

And by heavy, we mean the pressure is intense. Water is a lot denser than air, and that means the more there is of it, the more pressure is placed on whatever is inside. This means that deep water is very hard for people to go into - if you've ever swum in a really deep pool you might have noticed it hurting your ears and nose. This means people can't dive beyond certain depths, so exploring becomes a bit more difficult!