“The arctic rodents called Lemmings are well known for their periodic mass suicides by collectively swimming into the sea and drowning, or at least the Norway lemmings do. This is probably caused by environmental pressures usually from over population…”

—Mass SuicideHolology)

Actually, the cause of this behavior is more likely to be Disney employees hurling animals from cliffs in order to provide striking visuals for the 1958 movie White Wilderness. Another good Snopes find, which, among many other things, ruins an important metaphor in the classic IF game Trinity.