system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the heights of mountains measured against on planets without seas?

Like Everest is the highest on Earth at nearly 9km above sea level, but Olympus Mons, on Mars is the highest mountain in our solar system at 25km, but above what?
gpt
The Datum. Simply put, imagine running a bulldozer over the surface, smoothing down everything that sticks up, and using that stuff to fill in all the troughs and holes. The resulting surface is the reference.