My friend and I spent the better part of the day underground in Rat's Nest Caves, Canmore. What a great experience! We squeezed through the Laundry Chute, marveled at the formations, examined random bones and rappelled into the darkness. We barely noticed the hours slide by as we explored the darkness. Our guide, Kevin, was very safety conscious, professional, knowledgeable and just easy to be around.
I'd recommend this trip to anyone with a strong sense of adventure, reasonable amount of fitness... oh and maybe of a smaller stature!