The largest gathering of volcanic domes on the moon can be found in the middle of Oceanus Procellarum and close to the ring-plain Marius. Squeezed into an area of approximately 200 km in diameter the Marius Hills contains about 300 domes, half of the moon’s known population. The domes are just a few hundred meters high and thus best observed under low sun.
The sketch was made under a waxing gibbous moon, just a few days from full, and the sun angle was perfect for highlighting the many domes. Sketching them was another matter, quite cumbersome at that. Marius is the crater to the middle left in the sketch. Top right is the end of the Reiner Gamma swirl.
In 2009 the Japanese Selene mission found what might be a sky light to an underground lava tunnel in the Hills. The Marius Pit is located close to the rightmost dome in the sketch, but since it is just 65 meters in diameter it is well beyond the resolution of my scope. A fly-over movie from JAXA can be found here. Lava tunnels might be an option for a future lunar base, and apparently there is advanced plans for a private moon mission to target the Marius Pit.
When a lunar crater is formed by an impact, material from the depth of the crust is forced up and catapulted into the surrounding landscape, forming radial ejecta rays. The ejecta material has a higher albedo than the dark maria, making the rays plainly visible. Many lunar craters are associated with ray systems, most notably Copernicus, Kepler, Proclus and Tycho.
The Bessel Ray is a single ejecta ray running almost north-south through the middle of Mare Serenitatis. On it’s way it touches upon the Bessel crater (in the middle of the sketch), giving it it’s name.
Apparently there is some debate concerning the originator of the Bessel Ray: some suggest that it is associated with the nearby crater Menelaus (at the top of the sketch), others that it is part of the extensive ray system emanating from Tycho, far down south.
In an image from the Galileo spacecraft, the ray systems of Copernicus (left), Kepler (far left), Proclus (right) and Tycho (bottom) are prominent. Mare Serenitatis, with the Bessel Ray, can be seen near the center. Judging from the image the Bessel Ray seems to connect both with Menelaus (the bright spot on the southern shore of the mare) and with Tycho. I am not sure what to make of it, but if think I put my money on Tycho.