In the days of the ancient Temple, it was required on Yom Kippur for the high priest of Israel to enter the Holy of Holies and place sacrifices before the Ark of the Covenant. Unfortunately, the location of the Ark of the Covenant has been uncertain for well over two and a half millennium. Created sometime around the 14th century BC, it was the repository for some of the most important treasures of the ancient Israelites, including the original stone tablets of the Ten Commandments and original copies of the five books of the Torah. At some point during the First Kingdom period the Ark disappeared from both the Biblical narrative and from history. Its whereabouts have been the subject of speculation ever since. In all likelihood it was probably taken to Babylon as booty and ultimately melted down or destroyed. If it survived, these are some of the most popular theories concerning its current location.
According to the Bible, the Ark was placed inside of the Holy of Holies in the First Temple. This took place during the reign of Solomon, sometime in the 10th century BC. There is no mention of it ever having been removed, and as the most sacred object of the Israelites it is highly unlikely that they would have dared to do so. So the Ark was probably still there when the Babylonians showed up.
That said, it has long been speculated that somewhere beneath the Temple Mount a chamber was prepared to store the Ark and other Temple treasures just in case of an emergency. If this is true, the Ark could still be down there. Of course, there have been many efforts to find such a treasure chamber over the years, especially during the Crusades. If the Ark was ever buried in such a place, there’s a pretty good chance that it has already been found.
While it has already been stated that the Ark disappeared from the Biblical narrative, that is not exactly entirely accurate. A passage in the Book of Maccabees, which does not appear in most versions of the Bible, does suggest that the Ark was removed from Jerusalem by the Prophet Jeremiah prior to the Babylonian invasion and removed outside of the borders of Judea for safety.
The account further suggests that the destination was Mount Nebo on the far side of the Jordan. Mount Nebo is the location where Moses looked out over the Promised Land just prior to the Israelite invasion. This would have been a logical destination considering Moses’ connection to the Ark. It is supposedly buried somewhere on the mountain, along with the Altar of Incense and other Temple treasures.