Lemmings don't commit mass suicide. Some types, including the Norway lemming (whose habitat includes much of Scandinavia a nearby area of Russia) engage in rather regular mass migrations, due to a sudden population increase about every 3 or 4 years, during which many lemmings may accidentally drown if they attempt to swim (yes, they can swim) across a body of water that's too large.
I'm not sure if this mass drowning occurs because of misjudgement at the level of each individual, a migratory lead sub-group or individual, or some combination of those, or for some other reason altogether.
the 1958 Disney film White Wilderness, which won an Academy Award for Documentary Feature, [included] staged footage [showing] lemmings jumping into certain death after faked scenes of mass migration. A Canadian Broadcasting Corporation documentary, Cruel Camera, found the lemmings used for White Wilderness were flown from Hudson Bay to Calgary, Alberta, Canada, where they did not jump off the cliff, but were in fact launched off the cliff using a turntable.
There are many species of lemmings, including eight species of collared lemmings that are said numerous places online without citiation to be the only North American rodents that turn white in winter.
Image: "Lemmings in Migration", Popular Science Monthly, Vol. 11, 1877 (via the blog, The Scorpion and the Frog); click the image to view it at full size in a seperate browser window.