Ohalot, Chapter Fifteen, Mishnah Five
1) [In the case where] it was partitioned off from the floor:
a) If there is uncleanness beneath the partition, vessels in the house become unclean.
2) [In the case where] the uncleanness is in the house, vessels beneath the partition,
a) If there is a space there of one cubic hand breadth, remain clean;
b) But if not, they become unclean, since the floor of the house is reckoned as the house even to the nethermost deep.
Section one: In this mishnah, they build a new floor, thereby separating the house from its old floor.
If the source of impurity is below the partition, it travels up and defiles the contents of the house because the way of impurity is to escape from confined spaces. See also 3:7 for a somewhat similar situation.
Section two: If the uncleanness is in the house, then the purity of any vessels found below the partition depends on whether there is a cubit handbreadth in the area below. If there is such a space, then the impurity is found in an ohel and it is blocked from entering the house. However, if there is not an ohel below, then the vessels are considered as if they are inside the house. This is because the floor of a house is always considered part of the house, no matter how deep.