In Mas. Sukkah 20b, R' Yehudah states his opinion that for something to be an ohel enough to shelter Tumah (and create a situation where that Tumah is transmitted within) the shelter has to be man made. So he rules that a series of natural occurrences of shelters (as quoted from Mishna Ohalot 3:7) are not "Ohel."
But could they become an ohel if used as such even without any change in their structure? His proof for his position requires human participation in the creation, but the note in the Artscroll (#4 on 21a1) says "with the intent that it should serve as a shelter."
Does that mean that even without human construction, human intent can turn the natural hollow (smaller than Ben Avitach's fist) into an ohel? Can human involvement in creation be mental and not just physical?