A dark cavern whose walls are a static 2D signed-distance field — a few smooth-min chambers roughened by fbm, all analytic in the shader. Tapping fires a sonar pulse: an expanding ring lights the open air as it travels and, where the front sweeps across a wall, that stretch of rock flares and then lingers, phosphorescent, fading over seconds. The wall glow is the running maximum of each recent pulse’s ring-passage falloff, so echoes stack without any render target. Left alone a virtual explorer drifts through the dark, pinging every few seconds from slowly wandering positions in cold sonar blue.