In the non-mythos novel Earth's Last Citadel, Carcasilla is a city located in a vast cavern beneath a citadel of black stone on Earth in the far future after life was almost rendered extinct by the growing proximity of the moon. The name and a little of the description are reminiscent of Carcosa.
The citadel atop the city is located in a misty and featureless plain illuminated by the light of a dull red sun.
The cavern that hosts the city is illuminated by a blue/violet luminescence and irrigated due to the effects of the Tower of Rain. It is a place of beauty, built from coloured stone and crystal with many delicately-suspended edifices. However, the design of staircases and balconies seem to imply it was not meant for human habitation (it having been ruled by aliens at one point in the past), although it is home to a decreasing human population that has left it almost empty prior to its destruction in the novel.
The above information is taken from The Dictionary of Science Fiction Places by Brian Stableford.