Mare Smythii
This mare is situated between near (visible) and far sides of the Moon, like a link between known and unknown. That's why Johann Mädler named it after Alexander von Humboldt, a prominent explorer of unknown lands. It is one of only two lunar maria that are named after people, the other being Mare Smythii (named after British astronomer William Henry Smyth), and is located along the eastern lunar limb.