Polyethylene terephthalate, or PET, is a general-purpose thermoplastic polymer, and is used often in creating beverage and other bottles and containers. It's also a favorite of geocachers for hiding caches, particularly in it's "preform phase" which resembles a test tube with a soda bottle top. At the factory these preforms are heated and inflated like a balloon into the more familiar shape of a bottle, when the plastic cools it retains the shape and is ready for service.
The cache container in question is what happens when you heat one of these type of bottles again. Specifically when you put a reusable water bottle in the dishwasher. The result is a huge preform tube with a nice water proof top, awful for drinking out of but perfect for a cache. This hide should be an easy grab in an under utalized park in the city, an area great for a nice hike or a picnic and rife with other caches along the way.