Some believe that the Holy Ark is hidden under the mount based on the verse from 1 Melachim 8:8:
They extended the staves so that the tips of the staves were noticeable upon the Partition from the Sanctuary, but were not showing on the outside; and they remained there to this very day.
The words "to this very day" is believed to indicate that the Holy Ark is still hidden under the Temple Mount.