Oklahoma, formerly floating on a “Sea of Space”
It used to exist in another dimensional plane where the matter is denser and more durable than the world of Midgard (Earth), and is a place where gods dwell.
Journey Into Mystery #85 (1962)
Asgard is a small planetary body that serves as home to the Norse gods such as Thor, and their ruler, Odin. It exists in another dimensional plane and is about the size of the United States. It is not round like the Earth, does not spin on its axis, and does not revolve around the sun. It is a flat, asteroid-like mass that has a top surface with a gravitational pull, similar to that of the Earth’s, in order to keep the citizens and their cities from floating into the void. The matter on Asgard is also denser and more durable than the matter on Earth. There is an unknown force that keeps the surfaces of Asgard from eroding and from allowing its bodies of water from drifting off into space. Asgard has unknown intervals of day and night, it is unclear as to whether it uses Earth’s sun as a source of light, and there is no evidence that the seasons change.
Asgard is connected to at least two other dimensional planes that consist of the major land masses the Asgardians refer to as the “Nine Worlds”. Four of the Nine Worlds exist on the same asteroid-like mass that Asgard exists on. These worlds are Asgard, Vanaheim, Alfheim, and Nidavellir. Another of these worlds that is in a different dimensional plane is the world of Midgard, or Earth. The sixth world is that of Jotunheim followed by Svartalfheim, Hel, and Muspelheim. There are natural nexus portals that exist on each of these worlds except for Midgard, and some of these worlds are in direct contact with the roots of Yggdrasil, the cosmic ash tree that stands in Asgard.
The world of Asgard is the home of the gods while Vanaheim is the home of the Asgardian’s sister race, the Vanir. Nidavellir is the world belonging to the Dwarves and Alfheim is the world of the Light Elves. Midgard, or “Middle Realm”, is home to humans and does not seem to be affected by the motion of the other worlds of Asgard. Jotunheim is the world of the Giants and is also on its own dimensional plane hidden from Asgard or Earth’s. The only thing that prevents passage between Midgard and Jotunheim is Jormungand, the Midgard Serpent. The next world, Svartalfheim, is inhabited by the Dark Elves. This world is followed by the realm of the dead, Hel, and its sister realm, Niffleheim. Hel is ruled by the goddess, Hela, and is the home of the common dead. Niffleheim is the home of the dishonored dead such as murderers while there is a place known as Valhalla where the honored dead and heroes go. Valhalla is in a special place in the realm of Asgard. The final world is Muspelheim and that is the world ruled by Surtur and home to his fire demons. Muspelheim is on a separate plane of existence from all the other Nine Worlds and is said to predate recorded time as well as Niffleheim.
Asgard has one permanent portal and that is Bifrost, the Rainbow Bridge. That portal leads to Midgard, but it has been shattered making it difficult to travel between the dimensions. However, there are said to be nexus portals in the surrounding area of space that lead to Earth. Asgard also has a portal to Olympus which is home to the Greek gods, but since Olympus is not one of the Nine Worlds it is presumed that the portal is artificial. Asgard also has one more unique phenomenon which is the Cave of Time wherein one can travel to other time eras.
Besides the snake-like Midgard Serpent, there are three other single unique creatures that live on the various Nine Worlds. They are the winged dragon, Nidhogg, who lives in Niffleheim and gnaws on one of the roots of the Yggdrasil, Fenris, the giant wolf-god who is imprisoned in Asgard in Varinheim, and the ice giant, Ymir.
After a battle between Surtur, the fire demon, and Ymir which signaled the coming of Ragnarok, Odin endowed Heimdall, guardian of the Rainbow Bridge, with the Odin-power and gave Heimdall his scepter in order to perform a specific task. Odin described it as an "honor which is thine alone." Heimdall was given the privilege of restoring the Rainbow Bridge which anchored Asgard to Midgard once more. Also, when Thor became Lord of Asgard there was a brief period of time where Thor had Asgard occupy the same plane of existence with that of Earth.
In recent events, Asgard and all of its Nine Worlds except for Midgard seem to have been destroyed after the events of Ragnarok, the end of the cosmos in Norse mythology. However, the hammer of Thor, Mjolnir, was found in Oklahoma which led to the return of Thor. During his quest to find other Asgardians, Thor bought a huge piece of property where he erected Asgard on Earth. Iron Man confronted Thor about his construction, and after a short but heated debate, Stark proposed to Thor that Asgard would be considered a separate nation just like a foreign embassy. Thor agreed.