The San Diego Natural History Museum — known locally as the Nat — occupies the eastern end of Balboa Park's central plaza in a 1933 building that was expanded with a striking glass-and-stone addition in 2001. The collection covers the natural history of the Californias — both the US state and Baja California — with particular strengths in mineralogy, palaeontology, and regional ecosystems.
The palaeontology galleries are the best reason to visit: the California fossil collections are extensive, and the Cretaceous mosasaur skeleton near the entrance is a genuine exhibit rather than a cast. The biodiversity hall on the upper floors covers desert, chaparral, ocean, and forest ecosystems side by side. The museum also runs field research programs in Baja California that fund ongoing conservation work.
It sits directly across from the Fleet Science Center, and both can be visited in the same afternoon. The outdoor courtyard between the buildings is a good place to rest between visits.