Earth-One was the designation for an Earth, and the Universe it inhabited, that existed prior to the Crisis on Infinite Earths.
Earth-One itself was populated by various individuals who resembled those of its primary Multiversal counterpart, Earth-Two. Some were direct parallels to their predecessors (ex. Superman (Earth-One) and Superman (Earth-Two)), some less so (ex. Atom (Al Pratt) and Atom (Ray Palmer)). The two Earths were closely linked in terms of parallel development, although Earth-Two individuals usually predated their Earth-One counterparts by a few decades.
The primary superheroes were limited to the western heroes of the eighteen hundreds and few others, until Superman arrived from Krypton and ushered in a new age of heroes. On this Earth, Superman had a career as Superboy and teamed with the Legion of Super-Heroes in the 30th century.
From the perspective of the Earth-One Universe, the first travel to another parallel universe occurred when The Flash (Barry Allen) accidentally changed his vibrational frequency to match that of Earth-Two, where he met Jay Garrick, the Golden Age Flash. That meeting resulted in years of interaction between the original super-hero team, The Justice Society of America, and their younger Earth-One counterparts, The Justice League of America.
As Barry Allen was the first traveler, the honor of naming the two Earths fell to him - his universe's Earth was called Earth-One, Jay Garrick's became Earth-Two. As noted years later by the elder Superman, they (the JSA) were too polite to mention that they were around first .
As Allen explained to Garrick in their first meeting:
- "..I vibrated so fast -- I tore a gap in the vibratory shields separating our worlds! As you know -- two objects can occupy the same space and time -- if they vibrate at different speeds!
My theory is, both Earths were created at the same time in two quite similar universes! They vibrate differently -- which keeps them apart! Life, customs -- even languages -- evolved on your earth almost exactly as they did on my Earth! Destiny must have decreed there'd be a Flash -- on each Earth!" 
Forgotten Catastrophes: In the mid-20th century, Earth-One suffered several catastrophic events (while narrowly avoiding many more catastrophes), events which had no close parallels on "nearby" universes. Most or all of these were never mentioned again, in any subsequent story. These included:
- In mid-June 1940, Blackhawk Island was destroyed by Heinrich Himmler, with an ICBM-mounted nuclear weapon.
Future History: In the far future, criminals who had been found guilty of particularly heinous crimes were sentenced to be sent to the Earth of the 51st century, which was, during that period of history, a desolated planet. On one such occasion, a thief known as Mazdan was sent into this dystopian future via a golden time-capsule. However, after an accident occurred, he was sent into the past in stead, where, during the mid-twentieth century, the Flash foiled his robberies and sent him back to his future time period in order to be sentenced for his crimes.
Revised History: During the Crisis on Infinite Earths, Earth-One was destroyed. The New Earth that emerged after the Big Bang possessed elements of Earth-One's history, along with elements similar to that of Earth-Two, Earth-Four, Earth-S, and Earth-X. However, the events of Convergence restored it to the Multiverse.
Geography: A significant number of modern nations, secret kingdoms, and lost civilizations existed uniquely on Earth-One, and many of these had no direct counterparts on "nearby" parallel worlds such as Earth-Two or Earth-S. Examples include:
- Abyssia, an underground monarchy, accessed from a secret location in Switzerland. The ruling elite of this kingdom had telekinetic super powers.
- An unnamed, untouched medieval town, in the Swiss Alps, where no snow falls. The people of this town seem to be immortal, and their leader, Rafsu, has telekinetic super powers.
- Atlantis and Atlantis
- Dinosaur Island
- Paradise Island
- The Earth-One Universe is not known to have existed to the general populace of New Earth. Part of the Pre-Crisis Multiverse.
- Earth-Two superheroes appeared in Earth-One comic books.
- No trivia.
- Characters from Earth-One
- Other things related to Earth-One
- Earth-One's Comic Appearances
- Reality Gallery: Earth-One
Links and References
- Earth-One based on the article "Comments on a Finite Number of Earths" in the OFFICIAL CRISIS ON INFINITE EARTHS CROSSOVER INDEX by Lou Mougin and Mark Waid.
- Earth-1's heroes who were active during the Golden Age by John McDonagh
- Earth-One timeline from the Five Earths Project (Note: contains some data from fan fiction)
- Earth-One Chronology from Mike's Amazing World of DC Comics