The Marius Hills is a volcanic plateau, which is located at ∼13.4°N, ~55.6°W of the Moon. The name "Marius" is the name of the crater near the hills.
The Marius Hills are organized by several types of volcanic features, including domes (moutain with gentle slope), cones, sinuous rilles. So far, approximately 360 cones and 22 domes have been identified. The Marius Hills are a region where many volcanic activities occurred in the past.
In addition to the volcanoes, a deep hole (pit) has been observed on the Marius Hills' surface. This hole is located within a rille. The hole appears to have a lateral cavity at its bottom. However, it is not yet known how far this cavity extends. So far, no one knows the detailed internal structure of the hole.
The vertical hole of the Marius Hills. The diameter of the hole is around 60 m. It seems that a lateral space exists within the hole. However, it is not known whether this space truly exists or how large it might be. The image is from "Lunar QuickMap"
It is considered that this hole is caused by the collapse of a lava tube. Lava tubes are created when flowing lava cools and solidifies on the outside while the interior remains molten and continues to flow. If this hole is indeed related to a lava tube, we can know that intense volcanic activity occurred in the Marius Hills.
Mare Tranquillitatis is a lunar mare located at approximately 8.5°N, 31.4°E. Apollo 11 landed in Mare Tranquillitatis in 1969 and collected samples from the site. Many domes—mountains with gentle slopes of less than 5 degrees—have been observed in this region. These domes are considered to be volcanic in origin. Although the detailed formation mechanism is still unknown, it is possible that Mare Tranquillitatis was formed by numerous volcanic eruptions (it has also been suggested that Mare Tranquillitatis was caused by one large volcano).
Near the Cauchy crater, two faults have been observed, which are named as Rima Cauchy and Rupes Cauchy.
Image of the morphology around Cauchy crater. From LROC site for non-commercial use. (Credit: NASA/GSFC/Arizona State University).
As well as in the Marius Hill, there is a vertical deep hole(pit) in Mare Tranquillitatis. This hole is also considered to be generated by a lava tube.
Location of the vertical hole of Mare Tranquillitatis and Cauchy crater. The lunar image is from "Lunar QuickMap"
The vertical holes in the Marius Hills and Mare Tranquillitatis are very interesting spots for future exploration.
The vertical hole of Mare Tranquillitatis. From LROC site for non-commercial use. (Credit: NASA/GSFC/Arizona State University).