Deep Dark is a government organization which, very much like Department 13, maintains the mission of defending the United States from the unknown. Whereas Department 13 specializes in the supernatural, DD specializes in the paranormal, and normally can be found wherever there is a case that revolves around extraterrestrials or cryptids.
Deep Dark is housed in the Deep Dark Facility, another name for Area 51. Among their ranks are the entities known as the Men in Black, and a Horror, Xenos the Stranger, is housed at the facility. It appears to be an abandoned Air Force base from the surface, but actually descends 800 stories into the ground. Not much is known by ESPER abouot what lies beyond the first few levels, but it is reported that advanced alien technology including weapons and space vessels can be found there. Members of Deep Dark often can be seen using strange alien devices, weapons, and armor, and as such have proven to be a pain for ESPER agents attempting to investigate extraterrestrial or cryptozoological activity.