The Yoshimi Hyakuana, National Historic Site in Saitama Prefecture, Japan is a sandstone hill with ancient tombs that were carved between the sixth and seventh centuries. They were rediscovered again in the 1880s and were thought to be cave dwellings. In the 1920's though, more research showed they were burial sites.
It's name Yoshimi Hyakuana neans “100 holes of Yoshimi" but there is a lot more to the place than the name suggests. There are actually 219 tombs and the place has WW2 history as well. An underground aircraft-engine factory was built by between 3,000 and 3,500 conscripted Korean laborers. The war ended before production started.