We were also quite taken with this museum. Yes, the “flushing ball” you refer to is noteworthy. I couldn’t get over the system of signage, using the same street signs that correspond with those marking the intersections above ground.

Without even thinking, we emerged from the, um, bowels of this museum and headed straight to a lovely restaurant nearby. The maitre d’ gave us the once-over—an olfactory scan—and seated us at a remote table outside at a significant distance from the other patrons. It seems that we were bearing an aromatic vestige of our last stop. It’s not that the air down there stinks, exactly, but there is a distinctive odor—one we kept catching a whiff of for the remainder of our stay in Paris, each time we walked over a grill in the street. “Les égouts,” we would acknowledge, with a special nod.