A what?

Mike and I went out to run some errands while in Suffolk visiting his family. It had been raining for the past 2 days here. As we were driving through a rural neighborhood, Mike noticed some standing water in a roadside ditch. He commented that the ditch had poor drainage. I said, “Well, it has been raining for two days straight.” Mike: “Yeah, I guess it’s been a dormant vestibule.”