From Bulbapedia, the community-driven Pokémon encyclopedia.
The Eterna Historical Museum (Japanese: ハクタイ歴史博物館 Hakutai Historical Museum) is an anime-exclusive building located in Eterna City in Sinnoh. It made its appearance in A Secret Sphere of Influence.
Among the many artifacts inside the museum are a scale model of the Space-Time Tower and the Adamant Orb, the latter of which Team Rocket tried to steal while under the direction of Team Galactic. The Adamant Orb is no longer under its possession, as it was stolen by Team Galactic while it was being taken to Celestic Town for research in Losing Its Lustrous! via a police plane.
Also inside the museum are the many Sinnoh myths and legends.