This is an old one I actually made with Autodesk VIZ's scanline renderer. This was before 64 bit render servers and 8 gigs of memory! There is no way VIZ could render that much geometry. The trick is only one row of playgrounds is real. There are planes with a 99% reflective material along both sides of it. I set the ray tracer bounce 10 times. Part way through the animation the camera switches from hovering over real geometry to panning across a pre-rendered image.