This is the lake at the bottom of the Wallingford-Back mine in L'Ange Gardien, QC, about 20 minutes north of Buckingham off Route 309 (directions here on Google Maps). It was a feldspar mine, at one time the largest feldspar extraction site in North America, until larger deposits were discovered in Ontario. It closed in 1972.
What Google Maps won't tell you:
Chemin de la Mine is one hell of a twisty little gravel road that goes steeply up and down in the middle of the woods. I've been there three times in regular sedans, once in a minivan, and there was never a problem going in or coming out. This being said, I wouldn't try it in wet conditions unless you're driving something more rugged than a city car. Even in bright sunshine, you'll want to have good brakes, good tires, and a car in decent shape. Otherwise you'll be stuck in the middle of nowhere with no cell phone reception, looking at an extremely hefty towing bill.
You can drive all the way to the lower entrance to the mine. There's a lake at the bottom, but swimming or wading in is out of the question because the water is too cold even in high summer. There are some cool shots to be made at the lower entrance, but for the really funky stuff, you'll want to use the upper entrance. There's a trail to the right that'll take you around to the top of the mine. Look for a side trail to your left (it'll be the second one if memory serves me correctly). You'll see a large opening in the chain-link fence, and the mine entrance is impossible to miss. From there you can climb all the way down to the other side of the lake.
Other things to know:
It's cold in there. You can count on at least 5 degrees lower than outside, so it's a good idea to bring an extra layer. Climbing skills are not required; good hiking shoes are, and if you're going there for photography, bring the tripod. On my first visit I sort of managed at ISO 800, but it's a long way to drive for noisy pictures. An ND grad filter could come in handy as well if you're going to be shooting at the openings in the roof of the mine.
More photos of the mine on my Flickr page:
There is a wealth of information about the mine on this page (in French only).
This is a post from another blogger who visited the mine in the winter, with photos.
And this is a video of a dive in the lake.