I think that the area that was occupied by Rainbow Caverns is now partially occupied by the "mountain" part of BTMRR. But the mountain is mainly to the northwest of where the Rainbow Caverns building was. I'm pretty sure only the western half of the Rainbow Caverns land is now occupied by the Big Thunder mountain. It's unclear to me what they did with the eastern half of the space occupied by Rainbow Caverns. They may have just planted a bunch of trees. You'd have to compare aerial photos "then and now" to be sure. I think there was an aerial comparison done on this forum last year, around the time of the 50th anniversary of Mine Train thru Nature's Wonderland. There was also a thread showing the remnants of Nature's Wonderland which can still be seen today. If I was designing the BTMRR, I would have included the Rainbow Caverns, either at the beginning or end of the ride. You couldn't really use it for a lift section, since the caverns were all on one level. But it would have been a nice way to either begin or end the ride... and it was already built.