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Many places on the moon are named after "sea" and "ocean". The ancients looked at the moon and thought they saw the sea, but they were not. Some places on the moon have been knocked out of a big basin, and there are many cracks under the basin. The volcano erupted and some pits were filled more evenly. The black spots you see on the moon are actually mostly stones, such as black basalt.
Hongwan is a pit that was smashed out about 3.9 billion years ago. This hit splashed many stones and piled them to the side, forming a rainbow-like arc near the basin. So I have a romantic name-Hongwan.