Together with Mare Serenitatis, the „Sea of Moisture“ is one of the oldest lunar maria. The only 250 mile (400 km) wide sea of lava has flooded some older craters in the south. To the north it borders on the 55 mile (90 km) large crater Gassendi. In the northwest, the sea is bordered by the two 145 mile (230 km) long Mersenius-Rilles – two grooves, which, however, can only be seen in the telescope.