Not a question but a notice that the Naturhistorisches Museum in Mainz is currently closed. It's apparently reopening in summer 2019.
The friends we were staying with in Mainz suggested going there with our 5-year old son, but when we arrived, it was closed for renovations. Another friend even walked with us to the museum on the day, unaware that it was closed. A bit disappointing, as it looks quite interesting from the reviews on TripAdvisor.