The Heart of Atlantis is a crystal-based comet fragment that is used to power the city of Atlantis. It is generally shown as a glowing teal, rotating, translucent sphere with a jagged surface emitting a flaring white aura, and is seen levitating with stone effigies of the past kings orbiting around it. Its powers provide the Atlanteans with extended longevity and technological advancement.
Around 10,000 BC, a huge comet passed over the Earth. A piece broke off and landed in land belonging to Atlantis. Upon examination, the civilians uncovered the great properties it possessed. In time, they built their entire culture, civilization, and empire upon it. Since it is the sole source of their power, it is called the Heart of Atlantis.
The Atlanteans over-extended their power, and under the leadership of an arrogant King Kashekim Nedakh, they dominated other civilizations with ease. Then, an accidental weapons discharge threatened to wipe out the entire empire in one fell swoop. The Heart of Atlantis acted, bonding with the city's Queen, to generate a protective barrier around the center of the city. Those within would be saved, but would also be buried beneath the Earth. The time and power consumed to save the people resulted in the Queen being lost to the crystal forever.
Humbled and ashamed of how the loss of the empire and his wife came about, King Kashekim Nedakh had all past records of Atlantis destroyed and the crystal hidden deep within the city under the throne room. All knowledge of their history and the crystal had been virtually forgotten. Sometime later, however, a shepherd named Aziz accidentally wandered into Atlantis and spent two years secretly immersing himself in the culture, discovering how to locate the crystal and some of the properties it contained. He would record the information he called in a series of scrolls that would be converted overtime into the Shepherd's Journal.
In 1911, an expedition funded by Preston Whitmore and under the guidance of Thaddeus Thatch ventured to Iceland, where the Shepherd's Journal was successfully found. Three years later, Whitmore would fund another expedition, this time to following the directions supposedly written in the book that would lead the journeymen to Atlantis. Because of Thaddeus' passing, his grandson Milo Thatch would be the guide.
The expedition would reach Atlantis, finding to their astonishment that it was still alive and thriving. Sometime later, the expedition's commander Lyle Rourke proceeded with his attempt to take the Heart of Atlantis and sell it on the black market, not caring of the consequences doing so would result. Upon discovering the location of the crystal, it called to the King's daughter, Kidagakash Nedakh to be bonded with it. It is presumed that this was done to prevent immediate bloodshed.
Milo would lead an effort to stop Rourke from escaping with the crystal and succeeded. However, the battle that occurred caused a dormant volcano to awaken. Milo and the others manage to bring the Heart of Atlantis back to the city just in time for it to create a barrier that would protect the city from the flowing lava. It would then release Kida, having accomplished its task of saving the city. The Heart of Atlantis now once again hovers high above the city as a beacon of light and hope for the people.
Several years after the expedition, the Heart fuses with the Spear of Atlantis after the Team Atlantis claims the spear from Erik Hellstrom. With its power combined with the spear, the Heart was able to raise Atlantis to the surface for the first time in 8,000 years where the city would be currently located at a fishing spot in the Atlantic Ocean, out of sight.
The Heart of Atlantis contains mysteries properties that even the Atlanteans themselves cannot fully understand or comprehend. Through usage and wearing of a crystal piece, it extends the longevity of one's life by nearly five-hundred times the average human length. It also provides knowledge into technological advancement that is still, to this day, far superior to that seen on the surface.
The crystal appears to also possess a consciousness of its own, able to act and react when necessary. This is generally done whenever the city is unable to defend themselves against a certain threat. If it senses danger, the crystal will turn crimson and use beams of light to search for a member of the royal family (it won't accept any commoners). When it has found a royal family member, the crystal will turn back to its normal color, take control of said host's mind, bond with them, and use their energy to generate the power needed to materialize a force field which will protect the people.
When the crystal is enacting its powers, it glows brighter, and the king stones orbit it at a very high speed (which makes them appear as a glowing blue ring). The processing can expend the Atlantean royal to the point that they become lost to the crystal forever if the time and power necessary to consume is too excessive. Furthermore, looking directly at the crystal as the force field merges with it can cause irreversible blindness. With the Spear of Atlantis bonded with it later, the Heart gains the ability to raise Atlantis to its original location at the Atlantic Ocean.
The crystal is also conscious with regards to physical contact. If it feels that the holder is not of threat, it will do them no harm. Furthermore, the holder can use the crystal to heal wounds. However, if the crystal regards one touching it as a threat, it will poison them into becoming a crystalline being. While they would still be alive and conscious, they can easily be shattered, thus resulting in their immediate demise.
The Heart of Atlantis can also influence its surroundings. Through its powers, an entire ecosystem is created and generated so that the Atlanteans can survive beneath the Earth. This includes the production of water used to constantly cool the flowing lava below, preventing it from ever burning and melting the city. Flora and fauna are also influenced by the crystal, resulting in strange and unusual life forms to exist.