The Gallery is a 200-meter stretch of tunnel at Underworld depth 5, between junction markers D5-17 and D5-22. The tunnel is unremarkable in its construction — standard excavated limestone with concrete reinforcement at stress points, 3 meters wide, 2.8 meters high, ventilated by the standard Underworld air circulation system. What makes it remarkable are the images in the walls.

The stone surfaces of the Gallery display images. They are not projected, painted, carved, or applied. They are in the stone — variations in the mineral composition and crystalline structure of the limestone that form coherent, detailed pictures when viewed at normal distance. A geological analysis confirms that the images are an intrinsic property of the rock: the mineral variations extend through the full thickness of the wall, as if the stone grew this way. The images are as old as the limestone itself — approximately 430 million years, Silurian period. Limestone that is 430 million years old contains images of things that did not exist 430 million years ago.

The images depict scenes. Some are recognizable: a cityscape that resembles but does not match GLMZ. A face — human, specific, portraiture-quality — of a woman no one has identified. A machine that looks like a Behemoth but has too many legs. Others are abstract: geometric patterns of extraordinary complexity, shapes that seem to shift in the peripheral vision, compositions that create the illusion of depth in flat stone. The quality is remarkable. The detail is extraordinary. They look like the work of a master artist working in a medium no one has ever used.

The images change. Not while anyone watches — never while anyone watches. But between visits, images shift position, new images appear, old images vanish or alter. A portrait that faced left now faces right. A cityscape has a new tower. The geometric pattern has evolved. Security cameras installed in the Gallery record nothing anomalous: the images simply are different from one frame to the next, with no visible transition. They change between the frames, in the gaps between recorded moments, as if the images know when they are being observed and change only when they are not. The Gallery has become an informal art walk. Underworld residents bring visitors. There is no admission fee. The only rule, enforced by community consensus, is that you do not touch the walls. Not because touching is forbidden. Because the walls are warm, and they feel like skin, and that is more intimacy than most people want with a tunnel.