If you are interested in caves and their history, you have a treat here. Tham Lot is massive and some of it remains unexplored. A number of items (pottery, etc.) offer evidence the system was once the home of cave-dwelling prehistoric man.
A stream runs right through the mountain from one side to the other (roughly one kilometre) and it is possible for visitors to take rafts through the mountain to witness its various stalactites and stalagmites. If rafting concerns you, the trip can be done on foot, but it is a kilometre walk, so bear that in mind. It is a bit tricky and time consuming to get to, but if you can get there it is worth the effort.
Details: Local villagers act as guides through the caves and charge a ridiculously low fee of 100 Baht per group (so a tip might well be in order). Do not expect English commentary. It is possible to stay near the cave system as there is a camping site near the Tham Lot Nature Study Centre close to the caves.
How to get there: Tham Lot is located nearly 80 kilometers from Mae Hong Son municipality. Buses take you to a 9 kilometer-long road which leads to the cave system and from there a Songtaew will take you the rest of the way.