High Density Polyethylene, its just a fairly stable and strong plastic. That large farm pond linked is probably HDPE, the Rubbermaid stock tubs are likely a mixture of HDPE and something but I don't know for sure. Yeah, they are more costly than the intex sort of pool, probably cheaper in the long run, but we each have different goals. I used the intex last summer because I couldn't find the right solution. Also in case anyone is considering either of those links posted by Lukass, the difference is slight but the more expensive one has sturdier framing and a padding layer that goes inside the blue liner along the top of the pool. Probably there for its intended purpose so small children don't bonk their head on the frame and injure themselves. Also it has a slightly thicker liner, more akin to the stuff people use to line unground ponds although not nearly as thick as you would use for that purpose. The cheaper alternative has a thinner material with some sort of embedded fabric which probably allows it to have a similar strength. Also if anyone wants some slightly used, I have about 6 for sale in the Central PA area, real cheap. You'd need to pick them up as they are assembled.