This is one of the best rides in Massachusetts! Though very wet in the spring and fall, Bald Hill is a wildlife sanctuary with an incredible trail running around/through/near it. Follow the white markers on the trees for a 10 mile ride or so, but you can get quickly lost if you stray from the beaten path.. like me. Most of the intersections are numbered, which helps. The riding is spectacular, with technical sections (enough to stay interesting); there are wall rides, killer downhills and rolling paths that are well maintained, and scenery that will blow your mind in some places. Tons of wildlife and some nice climbs, too. Two interesting sites include a pair of car chasis that have been all shot up and a rock quary that used to be a Nike missile site (missile testing was done here in WWII). If you live in Massachusetts, you need to ride here! But bring lots of water and some food - a long ride and its easy to get lost.
Take 95 North to exit 51 (Endicott Road) and bear left at the end of the exit ramp. You'll see Middleton Road on your right very shortly - go down there about a mile and a half and you'll see the entrance on your left. It's a small entrance with a fire gate, a no parking sign, and a small message board. Just park off to the sides to avoid a ticket or towing.
http://www.singletracks.com/item.php?c=1&i=1506