The arctic soil is so cold that the ground beneath the tundra surface remains frozen all year. This permanently frozen ground is called permafrost. Each summer, when the sun warms the tundra surface, the top few inches of soil thaw. This melted part is called the active layer .
Plant roots grow within the active layer, and insects burrow here.
Farther south, the active layer thaws deeper. The active layer also thaws deeper beneath lakes, rivers, and roads because the water and bare soil absorb and hold more summer heat. Snow can fall in any month, but it doesn't usually stay on the ground until September. By June 1, the snow has begun to melt, but the active layer remains frozen. The sun remains above the horizon for 24 hours each day, and plants are producing new growth.