Once, while I was traveling on a vehicle in the Caribbean, I watched a pattern form over the course of about 30 minutes. The rendering below is an approximation of that pattern from memory. The gray color is true, and represents an entire surface that’s visible from the interior of the vehicle. The blue color is more symbolic than true, and depicts the approximate pattern that formed naturally on the gray surface.
To avoid giving away too much, I left out some details, such as which way is up or forward, where the seats are, and the size of the surface. I also simplified the surface to be square-shaped, when it's actually rectangular.
What is the vehicle, and how did the pattern form?
Update: I didn't anticipate so many different, feasible answers, so I will narrow it down with this additional info. The gray is the solid, opaque interior floor of the vehicle.