Wally's World (Walmart) sells tea tree oil in 2oz bottles for $7 or so last time i stocked up. the health-food store sells it for $13/oz. i'm the only one who uses it.
seeing this thread bumped i just read through it and enjoyed the stories. it just happens i'm in the middle of getting a large callous off my toe, and since it is in an area that i don't rub against anything there's no reason for that callous to be there other than the fact that sometimes warts hide under callouses.
some time ago, after a shower, i started cutting the callous off that area with my large toenail clippers and then i'd put tea tree oil on it. this past few weeks i've been cutting off the layers of skin to get down in there deeper and there i'm finally seeing the "roots" or "seeds" which are actually the blood vessels of the warts all scattered around that area. so bingo! theory supported by actual evidence.
now after getting to that layer of the problem i've been applying some wart remover and then after that dries i put some duct tape over it to keep it from getting on my socks. change it every other day or more. it's now cleared out almost all of the dark spots and looking much nicer. i will keep up this routine for another week or two to make sure i've got all the warts out of there and then see how it goes.
i think the problem with many larger or more difficult warts that keep coming back is that people stop too soon in the treatments. you have to keep going so that all the layers of skin underneath have time to grow back out and to make sure all the wart has been killed off (the virus in the skin cells that makes the warts appear) - this can take three weeks to a month. during this time make sure to be careful what you are touching because you can spread the virus and start up new warts. wash those hands.
my main use for tea tree oil is keeping the foot fungus from taking over. i use some tea tree oil and/or tea tree lotion between some of my toes after every shower and it will prevent the foot fungi from coming back, but at other times if i get a bad case i will use full strength bleach between the toes to knock it back along with even soaking my feet in hot water with some bleach in it to get all the dead skin off my feet. that is where the fungi lives and feeds, on dead skin. usually i get some in the summer from getting my feet wet too often and not keeping up after the fungi as i should. so by this time in the winter i'm usually just finishing up clearing it up again.
oh i also use tea tree oil on my toenails after i cut them to make sure there isn't any nail fungus getting a chance to get going. works.
but back to warts.

Mom has had plantars warts that have caused her pain, but within a few days of using wart remover and duct tape on them she felt no more pain and most of them went away. i've not checked her feet since, but i think we should start getting in the habit of doing that as i don't think she can really do a very good job of seeing them any more and bending that far. a hard thing to get her to let me do though. we'll see how that goes...