High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a polyethylene thermoplastic made from petroleum. It takes 1.75 kilograms of petroleum (in terms of energy and raw materials) to make one kilogram of HDPE. HDPE is commonly recycled, and has the number "2" as its recycling symbol.
Ultralight? Well, I guess it isnt heavy, but a disposable soda bottle or something would be a lot lighter. However, this bottle is extremely durable, to the point where Id call it indestructible. It also never leaks. Its a fantastic bottle, and I take it on every hike. I just have the plain clear one with the blue lid.