Wikipedia
Opelt is the lava-flooded remnant of a lunar crater on the Mare Nubium. It lies to the north of a similar formation named Gould, and to the northeast of the prominent Bullialdus.
Only a slightly raised outline remains of Opelt's rim, the interior having been inundated with lava. The rim has a wide break at the northern end and several smaller breaks to the south and southeast. The satellite crater Opelt E lies at the southern end of the surviving western rim. The interior floor is nearly level, with the exception of a slight rise to the southwest, which is continuous with a wrinkle ridge to the south.
To the north of Opelt on the lunar mare is a sinuous system of rilles designated Rimae Opelt. These occupy a diameter of about 70 kilometers.