Midgard is the realm of humans in Norse mythology. The name literally translates to Middle-Earth, a name that has been borrowed for the fantasy realm of the Lord of the Rings. The realm is called that, because it is situated at the center of the Yggdrasil tree and amid a large ocean.
Indeed, Midgard is said to be completely surrounded by water and is protected by the eyelashes of Ymir, which form a wall against any jötnar. In the water lives a giant snake, Jörmungandr, who has been dubbed the “world serpent”.
There is a rainbow bridge that is protected by Heimdall, which allows for travel between Midgard and Asgard. This bridge is called the bifröst. Both Midgard and the bifröst are destined to be destroyed during Ragnarok.