The Norse creation story tells us how the Norse cosmos came to be – from the hacked up body of a murdered giant. It's pretty weird, so let's take a closer look at the world that Odin and his brothers raise from Ymir's corpse.
Most of Ymir's body becomes the earth. Well, to be more specific, his flesh becomes the soil, and his bones and teeth turn into gravel and boulders and rocky outcroppings.
This earth is positioned somewhere in between the Ice- and Fire-Worlds – Niflheim and Muspelheim – that existed long before the gods' creation of Middle-Earth (a.k.a. Midgard, the world we humans call home). This new world is dangerously close to Muspelheim – sparks from the Fire-World can reach it. To protect it from these sparks, the gods raise Ymir's skullcap above the earth. The skullcap is held up by four columns guarded by the four dwarves called North, South, East and West.
Another threat lurks, though: giants. To protect humans from dangerous giants, the gods surround Middle-Earth with a wall formed from Ymir's eyebrows. (Hmm. Do you feel safer now that you know you're protected by a pair of enormous eyebrows? Yeah, we didn't think so.)
The mythological Norse world is home to more than just humans. After the death of Ymir, the giants form their own homeland, Jotunheim. The gods turn the maggots that grow in Ymir's corpse into dwarves, who live in dark caves and under the soil, and provide the gods with gold, silver, and iron. They also create elves, who inhabit the air, and nature-spirits who live in forests and streams. These races, along with all the animals and fish (and, eventually, humans), make the Norse world a pretty diverse place.
Many of these races have their own worlds, too. In addition to Jotunheim, there are two worlds for the elves, and Asgard, the land of the gods. These four worlds, in addition to Middle-Earth, Muspelheim, Niflheim, and the underworld, Hel, bring the number of worlds in the Norse cosmos to nine.
We thought you'd come sniffing around here for more on Ymir's body becoming the Norse landscape. We talk about that in "Symbolism, Imagery, Allegory." See you there.