As the leaves come off the trees, secrets are revealed. This wasp nest is bigger than a basketball, and will have residents again next year if it's not taken down.
Happily, it's several blocks away from my house, so not my problem. If I see the homeowners, I will point it out to them.
This is also why you want to make sure any bare wood on your house is painted. The wasps will actually gnaw wood to make the pulp for their nests. I discovered that on my old house, and had to have some piece of trim replaced. A coat of primer will protect the wood from both wasps and the elements.