It is a render of a randomly-seeded "Earth" that supports zooming. You can zoom in continuously to recursively expanding island formations. The formations are not random, in the sense that they are not decided on the fly. They are fixed based on the original seed and will stay the same if you come back to the same location. So the trick resembles displaying a map that takes up a huge amount of memory, even though that isn't really the case. As I recall the limits of the zoom have something to do with the width of floating point.

Apologies to Ken if I inaccurately described his demo. He usually makes the source available for download but I don't see how to get it here.