This nest has been in my garden shed for months, but before I removed it today (in case something else decided to take up residence), I thought I'd photograph it for posterity.  The wasp that made it would have left an entrance hole just big enough for it to get through so it's possible that something enlarged it to reach the larvae inside.  Apparently they're high in protein and low in fat, which makes them attractive to predators, but not to me just yet.  Around a year ago I posted a photo of a spider spinning a web and wondered how it knew how to do that, and now I'm just as amazed that wasps can build structures as complex as this.