Atlantis is an island first mentioned by the philosopher Plato around 360 B.C. It was said to have once been located off of the coasts of Spain in the Mediterranean and existed 9,000 years before Plato's time. Despite Plato's writings on Atlantis being the only known records of its existence he claimed that there were stories passed down by poets, priests and others before him mentioning Atlantis.

The reason that Atlantis fell and was lost to the sea forever was because the people of Atlantis started becoming greedy and power hungry, conquering other places in Africa all the way up to Egypt to add to their own before being countered by Athenians and being forced to retreat. As a form of punishment for the wrongs that the Atlantians had done, the gods set destruction to their city with massive floods and earthquakes which resulted in the sinking of Atlantis into the sea where it was never to be seen again.

El Dorado is said to be a city of gold where the people of the Muisca tribe live. According to the legend, this city of gold is located somewhere in Colombia. El Dorado means The Golden or The Gilded One. To the Muisca tribe, El Dorado was not a city but a king during a ritual. The ritual they performed would involve the man that was next in line to be king. This new king would be brought to lake Guatavita, stripped, and covered in gold dust. Next, he would go out onto a boat into the center of the lake with his attendants and precious gold. While out in the center of the lake the King would wash all of the gold off himself while his attendants would throw the gold into the lake as a sacrifice to their gods.

Shangri-La is mentioned in the novel "Lost Horizon" by James Hilton. It is said to be a valley hidden in Tibet and it's a place of all natural purity. This version is a more modernized invention, its actual origins can be dated back to sometime in the 16th century Monghul court. This version stated that it was a kingdom where all religions originated and where Christian communities lived.

Thule (also called Tile) is an island in the Arctic from Greek mythology that was first mentioned by a Greek explorer named Pytheas in his work called “On The Ocean” during his travels between 330-320 B.C. “giving the island a circumference of forty thousand stadia, and telling us also about Thule, those regions in which there was no longer any proper land nor sea nor air, but a sort of mixture of all three of the consistency of a jellyfish in which one can neither walk nor sail, holding everything together, so to speak”.

• Polybius in “Histories”

Agartha (also known as Agartta, Agharti or Agarttha) is a city that is said to be located at the very core of the earth. The entrance to this city is rumored by conspiracy theorists to be located somewhere in Antarctica. Shamballa is believed to be its capital city. According to Alexandre Saint-Yves d'Alveydre, Agartha will be accessible when Christianity lives up to the Ten Commandments “When the Anarchy which exists in our world is replaced by the Synarchy".

Camelot is a castle and court that is mention in the stories of King Arthur. Although, King Arthur’s legends is not where Camelot originated; it's actual origin is from 12th century  French romances. It didn't appear in the legends of King Arthur until the Lancelot-Grail cycle. It is said to be located somewhere in Great Britain although where exactly is still not known. One scholar named Norris J. Lacy said that "Camelot, located nowhere in particular, can be anywhere".

Definition:

In Norse mythology, Yggdrasil (world tree) is the center of the cosmos and holds all the nine realms on its branches. Yggdrasil was formed from the body of Ymir (the first giant). 

Name:

The name Yggdrasil means "Horse of Odin" according to Daniel McCoy:

"a reference to the time when the Terrible One sacrificed himself to discover the runes. The tree was his gallows and bore his limp body, which the Norse poetic imagination described metaphorically as a horse and a rider."

Realms locations: 

As for the realms within Yggsrasil it is not known as to the location of all of them except for Asgard, Migdard,  and Hel. Asgard is thought to be at the top of Yggdrasil, Midgard is at the tree's base  and Helheim is among the roots. However  the poem Grímnismál says that there are three main roots with one in Midgard, Jotunheim, and Helheim.

The Nine Realms:

Asgard: Realm of the Aesir gods and goddesses

Vanaheim: Realm of the Vanir gods and goddesses

Midgard: Realm of the humans

Jotunheim: Realm of the giants

Niflheim: Realm of ice

Muspelheim: Realm of fire

Alfheim: Realm of the elves

Svartalfheim: Realm of the dwarves

Helheim: Realm of the underworld

Animals of Yggdrasil:

Nidhogg: Serpent that gnaws at the tree's roots

Eagle: An unnamed eagle that sits at the top of the tree

Ratatoskr: Squirrel that runs up and down the tree carrying insults between the eagle and Nidhogg

Deer: there are four deer that eat the leaves of the tree, their names are: Dainn, Dvalinn, Duneyrr, and Durathror.

Definition: In Norse Paganism, Valhalla, which means "the hall of the fallen" is one of the places a Viking is believed to go after he dies in battle.

Pronunciation: Val-Hall-Uh

Background: Valhalla is located on Asgard where the gods live. This is where the greatest warriors go who fell in battle and are selected by the Valkyrie who also act as the waiters in Valhalla bringing the Einherjar (those who fight alone) their food and drinks.

Valhalla is described as having a gold-bright roof made of shields and has spears for rafters. The seats are made of breastplates that surround the many tables of the hall. The gates of Valhalla are guarded by wolves while eagles fly above it. During the day, Einherjar fight each other and during the evening all of their wounds disappear and they're back to full health. Every night during dinner they eat the meat of a boar named Saehrimnir who comes back to life every time he is slaughtered. Then, they drink mead from the goat Heidrun. Odin sits at the head of this table but gives all his food to his wolves named Geri and Freki.

The Einherjar who live here don't get to stay here for eternity as it might be in other afterlives. They are all chosen by Odin because they are to assist him in Ragnarök, which is the end of the world and means "fate of the gods" where Odin is destined to fight the giant wolf Fenrir.