When King Solomon built the First Temple he knew that it would not be the Final Temple. Subsequently, he built subterranean passageways and storerooms in order to store the Temple Vessels should the need arise.
The Talmud relates that the most important of the vessels were hidden away by King Josiah before the destruction of the First Temple (Tractates Yoma 62b and Horiyot 12a). The Holy Ark was Included among these vessels and was hidden in a place where it could not be found (Avot d'Rabbi Natan, Chapter 41 Mishna 12).
According to tradition once the Temple is rebuilt all of the Temple vessels will be returned to their rightful place.
