[Qqqwxs]

This comment made me curious.

Apparently, TAI specifically defines the second (in terms of cesium transitions) at sea level (where gravitational potential is equal). I never knew that second part.

[Xorathena]

...and one level of language-precision further, these articles get into how "sea level" differs from "where gravitational potential is equal":

https://en.wikipedia.org/wiki/Ocean_surface_topography

https://en.wikipedia.org/wiki/Geoid