The building at the North with the greenish roof.
A museum showing various historical treasures, most prominently a skeleton of a Dragon-type Pokémon (resembling Dragonite), is located in Nacrene City.
Just to the left of the skeleton is an Armaldo Fossil.
There is also a meteorite exhibit that will change Deoxys's Forme when examined.
Source: Bulbapedia/ Nacrene City