Along the west shore line there were some live cannon balls from that were left over from the Civil War. If you find one, use caution as they are sealed well and are packed with black powder which can still actually explode. I try to dive this site early in the spring when the water is still pretty clear before the algae grows. A mid-summer dive here can be like jumping into Pea Soup. I've been in this lake with a small two-person submarine down to 50 feet. It has been a nice testing ground for a friend's sub. The floor bottom is mainly mud that get stirred up easily, but the some of the shore line is rocky and large boulders which is fun to navigate around. Lots of tree blow downs along the east shore line, as well, and with caution you'll find it is interesting area to explore. Enjoy!
About 10 miles north on route 14 from Vermont's capital city of Montpelier is the little town of East Calais. In the Center of town turn left onto the Dump road and travel some 3 miles of dirt road to it's end. Turn right and follow to North Calais (which you will barely be able to tell you are in a town :), take the next road on the left which will take you right along the west bank of Mirror Lake (or #10 Pond). You'll come across the boat launch area first. It's my suggestion to park at the boat launch and take your first dive from the boat launch along the east shore line. You can enjoy depths from 15-40 feet just within the first 300 yards.