Opelt is the lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

-flooded remnant of a lunar

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 on the Mare Nubium

Mare Nubium

Mare Nubium is a lunar mare in the Nubium basin on the Moon's near side. The mare is located just to the southeast of Oceanus Procellarum. The actual basin is believed to be of Pre-Nectarian system, with the surrounding basin material being of the Lower Imbrian epoch. The mare material is of the...

. It lies to the north of a similar formation named Gould

Gould (crater)

Gould is the remnant of a lunar crater formation that lies in the midst of the Mare Nubium, in the southwest quadrant of the Moon. It is located to the east-northeast of the prominent crater Bullialdus, and south of the crater remnant Opelt....

, and to the northeast of the prominent Bullialdus

Bullialdus (crater)

Bullialdus is a lunar impact crater located in the western part of the Mare Nubium. To the north by north-west is the broken-rimmed and lava-flooded crater Lubiniezky. South-west of Bullialdus lies the smaller crater König....

.

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 craterlet 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.

To the north of Opelt on the lunar mare

Lunar mare

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...

 is a sinuous system of rille

Rille

Rille is typically used to describe any of the long, narrow depressions in the lunar surface that resemble channels. Typically a rille can be up to several kilometers wide and hundreds of kilometers in length...

s designated Rimae Opelt. These occupy a diameter of about 70 kilometers.

Satellite craters

By convention these features are identified on lunar maps by placing the letter on the side of the crater mid-point that is closest to Opelt.

Opelt	Latitude	Longitude	Diameter
E	17.0° S	17.8° W	8 km
F	18.1° S	18.7° W	4 km
G	16.8° S	17.2° W	4 km
H	15.8° S	17.3° W	3 km
K	13.6° S	17.1° W	5 km