The Planet Vortex
According to dialog in Ecco the Dolphin, the planet Vortex resides in the star
system Cassandra and crosses paths with Earth once every 500 years.
Vortex itself is based on mythology and a planet once believed to exist. The planet itself goes by many names: The Planet X, Nibiru. In some texts this hypothetical rogue planet is not a planet at all but a red dwarf star known as Nemesis.
No matter the name the stories all tell of a celestial body that has an entirely different orbit from the other planets and may possibly be apart of another star system.
The story of the rogue planet dates back to Babylonian mythology of both a god and a celstial body. According to Bohl, Nibiru was "The one who crosses the middle of the sea without calm..."
Atlantis
In Ecco the Dolphin, Atlantis was an advanced civilization
with crystals called glyphs that contained messages, activated
machines and granted access to barriers.
A famous psycic, Edgar Cayce best known as "The Sleeping Prophet" claimed to have the ability to look into peoples past lives. According to Cayce many people alive today are reincarnated Atlanteans. Cayce was able to channel information about Atlantis through readings with patients who were "former Atlanteans". He told of how Atlantis was devided into two factions called the "Sons of the Law of One" and the "Sons of Belial".
Cayce mentioned how the Atlanteans used machines that made use of special crystals as a power source. It is these same crystals that lead to three significant disasters; with the final plunging Atlantis beneath the sea.
Although Cayce spoke of Atlantis he did not create the legend we know of today. Cayce is known for popularizing an Atlantis that made use of advanced technology. The story of Atlantis originated in Plato's dialogues; Timaeus and Critias. Plato's version of Atlantis could actualy be a city in Thera that was destroyed in a volcanic eruption.
In Ecco the Dolphin, Atlantis was destroyed by a beam sent through space by the Vortex. The Atlanteans knowing an impending disaster was at hand used there time machine to escape into the past.
Sources: Nibiru (Mythlogy) - Wikipedia, Edgar Cayce - Crystalinks, Atlantis - Wikipedia