By any measure, pingos are remarkable features of the northern environment. These ice-cored hills, found in areas with permafrost, can continue growing for hundreds of years.

Chris Burn, a permafrost researcher, describes pingos as the landforms that are most like people. The Carleton University professor says that pingos experience conception, birth, growth rates and the effects of old age.

Every summer Burn visits Richards Island, located west of Tuktoyaktuk, to study a pingo that started growing there in 1978. As part of a long-term field experiment, researchers drained a lake on the island, simulating the natural event that Burn calls the point of conception for pingos.

After a very long pregnancy, a pingo was born at the site in the winter of '94-'95, and is now about a metre tall. If the pingo follows the normal course of events, its growth will slow down as it matures.

"Pingos are like people because they grow fastest when they are young and more slowly when they are older," says Burn.

On Richards Island, researchers have had the rare opportunity to study the complicated chain of events that creates pingos and other permafrost features and keeps them growing. The island is part of the Tuktoyaktuk Coastlands, located north of Inuvik, where about one-quarter of the world's pingos are located.

The Inuvialuit coined the term pingo, and most of the ones in their part of the world form on flat poorly drained terrain. These closed-system types can grow to be quite large, eventually resembling little volcanoes perched on the tundra.

Most of the Yukon's pingos are of the open-system type, which usually form in unglaciated valleys. Of the more than 400 pingos identified in the Yukon, the majority are located south of the Tintina Trench. Pingos rarely form on the Yukon's coastal plain because there are not many lakes there, and the ground is not sandy.

Open-system pingos form when groundwater flows downhill and is trapped underneath the permafrost. Eventually the water forces itself up through cracks into the permafrost and freezes, pushing the soil above it up into a dome.

Open-system pingos are not well understood, and it can be difficult to figure out the origin of a pingo just by looking at it. For example, no one knows for sure whether two pingos in the Blackstone Valley are both even of the same type.

A landform located near the Dawson City airport is often referred to as the Bear Creek pingo, but Burn questions the authenticity of this feature. A large lake has formed on top of this hill, and Burn does not understand how this could happen on top of a "live" pingo.

While open-system pingos are still poorly understood, more is known about closed-system ones, primarily because of studies conducted by J. Ross Mackay of the University of British Columbia, the world's leading authority on pingos.

Mackay began working in the western Arctic in 1951. His studies revealed that a pingo named Ibyuk, located just south of Tuktoyaktuk, is the world's largest growing pingo. It is about 50 metres high and continues to grow at a rate of about 2 centimetres per year.

Ibyuk is estimated to be more than 1,000 years old. Researchers can determine the age of a pingo by its rate of growth and by looking at the type of organic material growing on top of it. When lakes drain, the vegetation growing on the lake bottom changes and peat starts to form.

"If you can date the oldest peat then you know when the lake drained and the pingo started to grow," says Burn.

Burn says that a pingo pregnancy begins when a lake drains, allowing permafrost to form in the exposed bottom. Often a small pond is left on the lake bottom, and permafrost grows more slowly there compared to the surrounding area.

In the sandy soil of the exposed lake bottom, water expands as it freezes, pushing extra water ahead of the frost line. This water cannot escape through the layer of permafrost above it, and exerts a growing amount of pressure on the permafrost.

"The permafrost deforms at the weakest point, and the surface of the ground is pushed up. When that happens, a pingo is born," says Burn.

The forces that create the pingo can be substantial. "I have seen a cross-section of a pingo southwest of Tuk that has 25 metres of frozen sand on top of it. That would equal the weight of a building more than 20 stories high, so the doming involves substantial pressure," says Burn.

As the layer of permafrost becomes thicker, the pingo grows more slowly. As it pushes upward, a crack often opens in the top of the pingo. Eventually willows and grasses begin to grow in the crack, a sign that the pingo is mature.

"But some pingos keep growing and growing and growing," says Burn. As the crack at the top continues to widen, the sides of the pingo become steeper and erode.

Eventually the pingo loses its vegetation and becomes bald, a condition that can be fatal. As erosion continues, the exposed ice core of the pingo begins to melt and the pingo dies.

There is also an element of national competition on the subject of pingos, as Alaska claims to have a pingo on its North Slope even bigger than Ibyuk. But Burn says that scientists there do not know if it is growing.

"Their pingo could be dead. It may be growing and it may be bigger, but we don't know. We know the condition of our pingo, and we know that Ibyuk is vibrant," says Burn.

Chris Burn conducts permafrost research in the Yukon every summer. He can be contacted at crburn@ccs.carleton.ca.