Niðavellir

In 1937, Nazi sorcerers recruited into the Ahnenerbe from the Armenenschaft, Germanenorden, and Thule Gesselleschaft discovered another world, which they called Niðavellir after one of the worlds of Norse mythology. Their plans to use it - whether as a colonial frontier or source of occult wisdom - were cut short by ensuing events, and the parts of their research team that did not escape there were divided up by American and Soviet intelligence. Niðavellir became a secret front in the Cold War until both agencies, anticipating their liquidation, fled to it in 1979. It has been unknown to any major Earth government since then - although there have been rogue operations.

Niðavellir (known as Fairyland by the Special Activities Research Group of the CIA, Mir-2 by the 21st Directorate of the KGB, and various names by locals) is a disk floating in empty space, orbited by a sun and two moons. Water flows off the edge of it, but rainclouds forming around the moon, and spontaneously generated water rising from hot springs, keep its seas in equilibrium. Its similarity to high fantasy fiction we are familiar with both is and is not a coincidence - it developed independently of our world, but when Nazi sorcerers conducted the visualization exercises necessary to finding it, they were influenced by the same mix of Beowulf, Wagner, and Blavatsky that Tolkien and Howard were, and the world they happened on was one that echoed these imaginings.

Time passes more quickly here than Earth (on average - it's not an even rate), so it's known about 200 years of active intervention from our plane and another 200 of  mostly silence.