Mishna Keilim 10:1 discusses tsamid patil (צמיד פתיל), a tight covering for clay pots that protect their contents from external sources of tumah. It includes this statement:
היו כפויין על פיהן, מצילין כל שתחתיהן עד התהום. רבי אליעזר מטמא.
If they were turned over with their mouths downwards they afford protection to all that is beneath them to the nethermost deep. Rabbi Eliezer declares this unclean.
Thus, if a clay pot is upturned with its mouth to the ground (and then sealed with clay to the ground; see Yachin on the mishna), the ground acts as its tsamid patil, according to the first opinion. Rabbi Eliezer disagrees, and thinks that covers must be on the pot, not the other way around.
According to the first opinion, everything downward of the pot is protected from external tumah (עד התהום). Why should this be true? Does it only protect items that are in contiguous ground beneath it? What if the overturned pot was on the second story of a house, and there was tumah in the room below it: would it protect those items directly underneath it on the lower story?