Ohalot, Chapter Fifteen, Mishnah Seven
[With regard to] a house filled with earth or pebbles which he [decided] to leave there, or similarly a heap of produce or a mound of pebbles even such as Akhan’s mound, and even if the uncleanness is by the side of the vessels, the uncleanness cleaves upwards and downwards.
The mishnah discusses three situations:
1) Someone’s house is full of pebbles or earth and he decided not to remove them.
2) A heap of produce that is found outside of a house.
3) A mound of stones, even if it is like the stones that were put on top of Akhan when he was executed (see Joshua 7:26).
In all of these cases there is a source of impurity within the mound (or house) but it is not touching the vessels.
The mishnah rules that the impurity goes up and down but does not defile the vessels. In other words, the impurity is not found in an ohel, and therefore it doesn’t spread to the sides.