Now for some good stuff - a topographical map of the planet Mars arranged by altitude. Notice the southern hemisphere is so much higher!
There are some points that go way, way higher than 8km on Mars, but that's because this picture is what the scientist Gauss calls a Geoid - a crucial concept in planetary mapping and geodesy, a sort of surface model/figure of the earth (or other planets) that smooths out extremes by surface gravity. However, the use of 8km as a base shows it goes by earth gravity - which may result in a less useful model.
A geoid is very different from say, a reference ellipsoid, which is a smoothed out, idealized diagram of the earth. These two are used together in the field of Geodesy, which is scientific surveying, how maps are constructed and GPS works. It is the use of mathematics to determine altitudes and points on a map.
If you're like me, you're the sort that gets curious and asks questions, and one for me is this: how exactly is that they KNOW that Mt. Everest is the highest mountain on Earth? The answer is that it wasn't known. Until 1849, it was commonly believed that Kangchenjunga in North India was the highest mountain in the world (today, it is believed to be the 3rd tallest).