Narcisse Snake Pits
The Narcisse Snake Pits are located in the Rural Municipality of Armstrong about six kilometers north of Narcisse, Manitoba, Canada. The dens are the winter home of tens of thousands of red-sided garter snakes (Thamnophis sirtalis parietalis). These pits are the largest concentration in the world of this particular type of snake. Their winter dens are subterranean caverns formed by the area's water-worn limestone bedrock. In the spring, they come up from their dens to the snake pits, where they engage in mating rituals. Then they disperse into the nearby marshes for the summer.
The population of red-sided garter snakes around Narcisse was roughly 70,000 until terrible weather in 1999 killed tens of thousands of them before they could reach their winter dens.  This tragedy triggered concern about the snakes' biannual migratory path, which cuts right across Highway 17. Every year, ten thousand snakes trying to get to or from their winter dens had been crushed under the wheels of vehicles. This had not been a problem before, because the vast population compensated for the losses. After the winter of 1999, however, the population of garter snakes was dangerously low, causing Manitoba Hydro and volunteers to intervene.
Foot-high snow fences were built to force snakes into 15-cm tunnels that went under Highway 17. Since some snakes still managed to squeeze under the fence and onto the road, signs were put up during the migratory season urging motorists to slow down to avoid accidentally driving over snakes. These measures worked, and now less than a thousand snakes per season are killed on the highway.
The conservation area is open to the public. The snakes are most active during the spring and fall - in late April to early May, which is the mating season, and also in early September, when the snakes slither back down to their winter dens. 
- Rae Bridgman (author of The MiddleGate Books, which feature the snakes of Narcisse, Manitoba)
- The Snakes of Narcisse | Wildlife and Ecosystem Protection (Province of Manitoba)
- Snakes Alive! (Nature North Zine)
- NatureNorth's - Narcisse Snake Dens Videos