It is not proved that this room is the oldest part of the villa.
Debated. The underground room is only believed to come from before the emperor's villa.
It is at Hadrian's Villa, near Rome, in the Palazzo area. It dates to the Republican era, before Emperor Hadrian built his villa in the second century AD. The room is a hypogeum. It may have been a silo or a store. It was left, and partly filled with building rubbish and pottery. The finds include terracotta pieces with animal shapes.
The page is not sure the room was a store. It calls this the only sure Republican find at the villa so far.
https://www.ancient-origins.net/news-history-archaeology/hadrian-villa-hypogeum