The large, darkened circular area in the upper right of this image is "Mare Humorum" (the "sea of moisture") on the lunar surface. The flat plane containing few craters is thought to have been produced by lava flowing over the sea bed. There are several "seas" or maria on the surface of the Moon, which are darkened areas that were first detected with the invention of the telescope in the early 17th century. Astronomers initially speculated that these "maria" represented seas on the surface of the Moon.
The most notable feature of the Mare Humorum is the large crater Gassendi at the bottom of the sea. Gassendi (70 miles in diameter) is touching a smaller crater (Gassendi A), and exhibits a peaked center.
Beneath Gassendi is another sea, the Oceanus Procellarem ("ocean of storms"). About a crater length below Gassendi is a small, distinct, circular crater. This is Herigonis. With a diameter of only 10 miles, it can be used to calibrate the width of some of the smaller craters.