You would want at least a mountain on (or "behind" instead of "on", with its tip...

Aardwolf on July 27, 2017 | parent | context | favorite | on: Longest Lines of Sight on Earth

You would want at least a mountain on (or "behind" instead of "on", with its tip sticking out) the horizon as well. You could see a distant 6km high mountain from another 6km high mountain much farther, than flat horizon from an 8km high mountain (mt Everest)