It's a cool adaptation of the effects of paper marbling, which you can do at home with amazing results. A friend at work has recently gotten really into paint pouring, which you can see here and here.

Lovely. I remember when I was a kid going on a school trip to an art school where we watched someone making marbled paper to use in bookbinding. I still think of that day every time I see an old hardback book with a beautifully feathered flyleaf.