According to Jeremy Clark, we now know of one more Charlemont ski area. Here are the details, from his discussions with multiple sources:
Many local skiers are aware that the Berkshire East Ski Area was once called Thunder Mountain. However, Berkshire East as we know it opened in 1960. Four years before that, the original Thunder Mountain opened, operating in only a small part of the current Berkshire East Ski Area.
In the mid 1950's, Arthur Parker came out on a ski train that was going to Chickley Alps Ski Area, down the road in Hawley. Well, this trip inspired him to build his own resort on Mount Institute. And so it happened, Thunder Mountain opened in 1956 with two rope tows and a few short trails. The vertical drop could not have been more than 200 feet. The ski area lasted only about two years, as the snow never really came. Still, this didn't hold back Parker, as he came back a year later and built a bigger, better Thunder Mountain around this existing area. However, this version of Thunder Mountain isn't a lost ski area, as it is now Berkshire East.
More information on the first Thunder Mountain. It was located on either side of the last stretch of Mohawk. One rope tow was located above the Deerfield River Club Condominiums. If you walk around in the woods above them, you will see the old wheels from the lift, along with smaller trees. The other rope tow was located between Big Chief and Mohawk, above the original lodge, which is still standing. This trail is still recognizable, as the brush has not grown too tall yet. The main office at the original Thunder Mountain was located where the old ski patrol building was, the current location of the Bobcat trail and rope tow.