The stream of water soon stops, and the bottom of the cube closes itself. Meanwhile, you readily find a bridge which is about to dock with the center island, and wait for an opening to appear.

And wait.

And wait.

All told, roughly a quarter of an hour passes without any sign of an entrance appearing. Finally, though, you feel the rumble as the complex starts to shift again. This one appears to be a more dramatic movement than the last, with even some of the islands swinging about; the cattle ranch you visited earlier is now on the opposite side of the mountain.

The bridge under your feet doesn't seem to be involved in all this movement, but there's still a dramatic change in your surroundings. As the shift winds down, a door does open in front of you. Not just any door, though, but an enormous one, built to the same incredible scale as the complex itself. The opening is easily a hundred feet across, and equally high, as a huge section of the mountain slides to the side.

Inside, you see a cavernous space. The area is as large as the door, and then extends for another thirty feet below your height as well. That entire space appears to be filled with a complex device for manipulating light. The sunlight streaming in the door is captured by lenses and funneled into a network of lenses, prisms, and mirrors which capture and direct the light. Figuring out exactly what the device is doing will clearly take some time, but you can see the most obvious results. First, various patches and beams of light are thrown around the room, leaving the walls, ceiling, and floor in a chaotic arrangement of darkness and intense, often colored light. Second, a very bright, very green beam of light leaves the device and goes into a particularly large lens set in the wall opposite you.

Spoiler: Perception DC 15
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You can faintly hear the sounds of machinery coming from underneath you. It sounds like it's coming from under the floor, to your right.