Although I had photographed Mare Humorum before, I dont remember if I ever photographed it in its entirety. This image uses parts of 4 frames to create the entire mare and surrounding area.
Mare Humorum is 391km in diameter and covers 113,000 square kilometers. It is a large feature, but is one of the smaller mare's. If you look real close along the eastern edge of the mare, you will see a 2nd ring a little farther east. It is thought that the mare's are very old impact craters from tremendously large chunks that struck the moon early in its formation. The dark floor was caused by the tremendous energy created in the impact, and what you see is molten rock (lava) caused by the energy of the huge impact.