The barrels that receive the sap coming down from the tapped trees. It's hard to see in this photo but there are blue hoses snaking through the woods from tree to tree, leading down to these barrels.
I'm noticing LisaNH, one of my contacts, has further shots in her photostream showing the same.
www.flickr.com/photos/lisanh/2291921739/in/pool-maplesyrup