That is the inspection hole giving access to the timing marks on the flywheel, as others have noted. It opens into the clutch bellhousing, which has no oil in it, and indeed is open at the bottom.
There should be a bung looking like a hot-water bottle top in there - flexible plastic with a big tab on it so you can pull it out easily. With it missing there is a small chance that something solid will drop in there and jam against the flywheel or the clutch mechanism. That would be most likely to damage the gearbox casing. Worth replacing for the small cost.