system
Given the following post, respond as if you're commenting on Reddit:
human
How are the oceans sealed? Why doesn't all the water leak out the bottom and go deeper into the earth?
gpt
The oceans are made up of about 8-10 km of igneous rock. There's a whole bunch of deep gabbros, and feeder systems, which then cap out with erupted material forming pillow-basalts surfaces. In time this all gets draped with sediment. It's important to note that when magma moves up through any crust, it passes up through fissures and cracks that it forms on the way, and these are left filled with residual material at the end which solidifies, so there are no great voids or air pockets etc.

Water certainly does percolate and penetrate into oceanic rock. However, igneous rock is not highly permeable, so the volumes of water a pretty small, and indeed those volumes get ever smaller with depth, as the mass of the rock above ensures that pore spaces get smaller and smaller with depth.  Consider, too, that the plates sit on top of 1200 degree mantle, so there is a temperature gradient through the crust - you don't have to go very deep before the temperatures are hot enough to drive buoyancy-related fluid motion, so water cycles through the upper portion of the crust, rather than just forever penetrating deeper.

The water that *is* in the oceanic crust is really important geologically though, as when the ocean plate eventually gets subducted, this water content gets introduced to the mantle, and causes partial melting of the mantle rocks. That is what helps generate the magma at subduction zones which creates subduction volcanism.