I came across this tree with a hole through it while I was hiking in the Ouachita National Forest here in Arkansas this past December. I spend most of the winter hiking this forest, and so I get to see lots of cool looking trees.
I usually don’t like being in my photos but I thought this would give you an idea of the size of the tree and the hole.
Hollows may form as the result of physiological stress from natural forces causing the excavating and exposure of the heartwood. Forces including wind, fire, heat, lightning, rain, attack from insects (such as ants or beetles), bacteria, or fungi. Also, trees may self-prune, dropping lower branches as they reach maturity, exposing the area where the branch was attached. Many animals further develop the hollows using instruments such as their beak, teeth or claws.
Hollows are an important habitat for many wildlife species, especially where the use of hollows is obligate, as this means no other resource would be a feasible substitute. Animals may use hollows as diurnal or nocturnal shelter sites, as well as for rearing young.Wikipedia contributors, “Tree hollow,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Tree_hollow&oldid=895774125 (accessed June 27, 2019).