Earth-90

"Barry, where's there?" "My Earth, Earth-90. I've traveled from there to warn you."

- Oliver Queen and Barry Allen

Earth-90   is one of the many "Earths" in the multiverse.

History
In late 2018, Mar Novu/The Monitor ravaged the Earth of this universe and then used the Book of Destiny to massacre all its inhabitants, leaving Barry Allen as the only known survivor.

From The Monitor's realm

 * Mar Novu

Locations

 * United States of America
 * Central City
 * Chicago
 * California
 * New York
 * Arkham
 * Paris
 * Great Britain
 * Great Britain

Trivia

 * It is similar to both Earth-1 and Earth-38 due to the existence of real-life figures such as celebrities and politicians, law enforcement, crime, vigilantism, government, history, historical events, holidays, politics, political issues, life, everyday life, physical laws, nature, science, popular culture and technology unlike many other Earths.
 * The I.R.S. (Internal Revenue Service) exists on Earth-90.
 * Earth-90's existence would seem to contradict Kara Danvers' statement that only 52 Earths exist in the multiverse (53 including Earth-X). For this reason, it is unknown if infinite universes truly exist in the multiverse or Kara's belief is a continuity error.
 * Both Arkham and Lois Lane are referenced in the series, suggesting that Earth-90 may also have a Batman and Superman.
 * Gorilla Grodd is mentioned as a foe defeated by Nightshade, pre-Flash. It's not made clear whether he's actually a gorilla.
 * When asked if Flash had been built in a lab, Barry jokingily hypothesized that a Professor Zoom was behind the hero's origin. He then had to pretend to be a mad scientist character based on the joke.

Behind the scenes

 * The number 90 is a reference to the original 1990s television series,.
 * Marc Guggenheim later confirmed on Twitter that the '90s series actually takes place on Earth-90.
 * The dead warriors of this Earth, while not named, depict various versions of DC Comics characters. These include Stargirl, Hawkgirl, Huntress, Hawkman, Green Arrow, Captain Cold, Firestorm, Ray, Jesse Quick and Obsidian.
 * Real-life similarities (all of which has been mentioned and/or seen) that have been confirmed to exist on Earth-90 include people such as Elvis Presley, Madonna, and Mother Teresa.