Eli5 : if pressure is due to the size of the column of fluid above, why is it not lighter inside ?

Question

I know that if you are inside of a hermetic room, the pressure is the same. But I can’t understand *why*. For example, if you enclose water from the bottom of the ocean in a box, shouldn’t it be suddenly at the same pressure than outside, due to the fact there is not the weight of the water column above ?

Thanks!

In: 2

Answer 1

The water column weighs down on your enclosure, which in turn pushes on the water inside it. Only when you remove the enclosure from the bottom of the ocean, can the water expand and relieve the pressure. It then pushes on your container, which either expands as well, or pushes back on the water if it’s rigid enough.