An opening in the Earth's crust where water is expelled is called a hydrothermal vent.

Here's why:

* Hydrothermal refers to hot water, which is what is expelled from these vents.

* Vent simply means an opening.

These vents can be found both on land and in the ocean, and they are a fascinating example of how the Earth's internal processes interact with its surface.

Let me know if you'd like to learn more about hydrothermal vents!