Permafrost is permanently frozen soil and you can find it underneath the entire North Slope of Alaska. It can be about 90-600 meters thick here, but near Toolik lake it is 200 meters thick. Permafrost thaws every summer and affects how deep roots can grow and therefore what kinds of plants can grow. That's why we don't see plants that need deep root systems. (Read more about that in our post about trees.)
People who live in this area are always thinking about permafrost when they build a house, road or airport runway. If a large amount thaws it could cause soil collapses, which will destroy any structures that have been built.
Have you noticed how swampy this area can be? That is because the water in the upper layer of soil that thaws every year, the active layer has no where to go. The permafrost below acts like a barrier. This is why we need good shoes when going out into the field. It it a bit soggy. Measuring the depth of the permafrost is pretty easy. We use a thaw depth probe, which basically looks like a giant metal capital T. Some of them have the centimeters marked on the probe, others require that you take a ruler with you into the field. You push the probe into the ground until you feel it hit ice, or permafrost, mark the spot where the soil is, pull it up, then read off the distance between the bottom of the probe and where the top of the soil was. This is typically done in two spots in a plot or twice at a set distance along a transect. Watch the following video to see Hannah and Becca use the thaw depth probe!