The Nine Unknown by Talbot Mundy

A pale-green astral-looking light developed gradually, turning the heart of darkness into twilight. They discerned the shadowy outlines of a cave buttressed with titanic masonry. There were no images, no carvings on walls, nor anything to mar simplicity. The proportions expressed restful, pure and final peace.

There was no smell of dampness, although Ghandava said they were under the bed of Ganges. There were no bats, no filth, no occupants. There was nothing in there--in an acre of earth's foundations--but one square altar set against a wall; and thence the light came, seemingly.