It was discovered when a experiment with electricity went terribly wrong. Fifteen workers were killed with a bolt of electricty came from one of the --Classified-- The intense energy arched between the walls of the gateway and created an opening into another and mysterious world.
The Midnight Syndicate kept this secret from the world with only few people allowed to know anything of it's existance. The Syndicate sent explorations in to the strange world. They found many structures and signs that there used to be life here, but they never found anything alive. The earliest picture is dated from before 1925. The exact date is classified to all but the highest members.
The Syndicate requested the help of the Organization called the Black Guardians. They did their investigation to strange symbols they found in the biggest monuments They didn't find any earthly connections to any of the symbols. The Guardians ended their investigation learning only three things.
Their suggestion was to not open the Dimensional Gate again for the safety of the world's integrity intact.
The Dimension was decided to be closed, never to be opened again. That choice was reversed when the first time the Delta Squad came up against a villain that was beyond their ablility to fight the Syndicate order the villainess Amanda to be forced in there. She was imprisoned there and the Squad declared victory. From that moment on the Dimension was declared to be a prison for extraordinary forces.
The Dimension itself is a dead world. It has signs of a civilzation long since destroyed. There is always signs of light, but never any sign of the sun. The wind always sounds like it's blowing but nobody has ever encountered any moving air. The ground is all the same color, it's black as if it was charred by fire. The Guardians' information has been kept top secret since it has come out from the gate. Nobody knows what it is to this day.
Time moves differently in the Dimension. People who are trapped in here remain the way they are for as long as they are inside. They don't age while in the alternate world.
There is only one truly known exit back to the living world. The scientists were able to make it only work for the Delta Squad members due to their unique life energies they obtained on the island. It works for nobody else unless one of the Squad members is in physical contact with the person they want to bring out at the time they leave.
The place is still largely a mystery, even to the ones who have direct access to it.