system
Given the following post, respond as if you're commenting on Reddit:
human
What does mean sea level mean if you're thousands of miles from the sea?

As I understand it, Mean Sea Level is defined by taking the time-averaged effective gravitational potential of the water-air boundary averaged over the world's oceans. Since sea levels are rising globally, we choose some datum and stick to that. This seems reasonably unambiguous.

If the summit of Everest is 8848m above MSL, does that mean it's 8848m directly above a point within the crust on that same equipotential, or does it mean it's on the equipotential defined by the average of all points 8848m directly above the sea? Or does it mean something else?

How much difference would there be?

*To rephrase the question: if you hollowed out Everest and made tunnels (sealed against groundwater) connecting that cavity to the oceans, would the water find a time-averaged level 8848m directly below the summit?*
gpt
Sea level is affected by gravity/mass (hence the moon causing tides and so on...). Therefore mountains affect the sea level. Currently, sea level is calculated by assuming the entire earth is covered in water, but the mass from the mountains is still there (hard to imagine, i know). Then the water "bulges" in some places where there is a lot of mass (also tectonic plates and such play a role), in these places sea level is "higher". To answer your rephrased question: yes (assuming that the tunnels do not measurably change the mass of Mt. Everest).