By their very nature, secret bases are not spoken about openly. Officially, they don't exist, or at the very least, the activities taking place at certain locations are vastly different than what is officially stated.
While the vast majority of this list may need to be read with a pinch of salt, the following claims are nonetheless intriguing and most definitely alluring to the part of one's brain that, in terms of the authenticity of the claims, asks the question, "but what if . . . ?"
10 Base AL/499
Most of what we "know" about Base AL/499, claimed to be 60 meters (200 ft) below the small English village of Peasemore in the Berkshire countryside, comes from apparent whistle-blower James Casbolt. Casbolt claims his family has both high-ranking intelligence and Illuminati connections, and because of this, he says he was "born into" a top-secret program that operates out of the underground facility, called Project Mannequin.