Though it's a distance from the South Rim of the Canyon (120-ish miles), the caverns are definitely worth the trip. Take a tour into the caverns, where there is a "hotel" room and survival rations from the 1961 Cuban Missile Crisis. Apparently, JFK had food and such stashed in the caverns in case of emergency. Aside from the nearly 50 year old cans of water are fossilized animals, amazing rock formations, and amenities to spend a comfortable night underground. If we had time, we would have stayed, but had to continue on our journey West.