Tundra is the huge flat land in the far north — northern Russia, northern Canada, Alaska, parts of Scandinavia — where it is too cold for trees to grow. Picture the map above the forests and below the permanent ice: that band is tundra. The ground there is frozen solid a little way down, all year round, and that frozen layer is called permafrost. Roots cannot push through it, so nothing tall survives. What does grow is low and tough: mosses, lichens, small hardy flowers, tiny bushes that stay close to the ground where the wind cannot kill them. For most of the year the tundra looks brown, grey, and empty. Then in the short summer the top of the soil thaws, the water cannot drain away because the layer below is still frozen, and the whole place turns into wet ground full of pools, flowers, and enormous clouds of mosquitoes. Reindeer, caribou, arctic foxes and huge numbers of birds arrive to use those few warm weeks. Grammar note: tundra is normally uncountable — you say "across the tundra", not "a tundra" or "three tundras". Scientists sometimes write about alpine tundra, meaning the same treeless, cold conditions found high up on mountains rather than far north; that is the one place the word travels away from the Arctic. You are most likely to meet this word in geography class, in nature documentaries, and now very often in news about climate change, because as the permafrost warms it releases gases that were locked in the frozen ground.