ANSWER: The phrase "Ark of the Covenant" occurs at least forty-three times in the King James Bible. Not suprisingly, most of its occurences are found in the Old Testament. Although one might expect the books of Numbers or Deuteronomy to directly refer to the ark the most, the writing that references it more than any other is the book of Joshua (Joshua 3:3, 6, 8, 11, 14, 17, 4:7, 9, 18, 6:6, 8 and 33). Direct reference to the Ark of the Covenant is found, only once, in the entirety of the New Testament (Hebrews 9:4).
In studying this fascinating subject on the Ark of the Covenant, it should be noted that the King James Bible refers to this sacred object of Israel in several different ways other than what was mentioned above. It is called the Ark of the Testimony (Exodus 30:6) and the Ark of the Lord (Joshua 4:11,1Samuel 4:6). It is also referred to as the Ark of God (1Samuel 3:3, 17, etc.), of the God of Israel (1Samuel 5:10) and the Ark of God's strength (2Chronicles 6:41).
The Ark itself was a very special container Moses built according to the specifications given to him directly by God (Exodus 25). It contained the stone tablets (given to Moses) which delineated the Ten Commandments (the basic terms of obedience from which the Old Covenant was built), the rod of Aaron the High Priest that budded, and a pot containing manna. Its first mention in the Bible is in Exodus 30.
The Ark was the holiest physical object located inside the holiest place in God's temple. Placed in an area commonly referred to as the 'Holy of Holies' or the 'Most Holy Place,' it represented the very throne of God. The room where this object resided was so special that only the temple's High Priest, once a year (on the Day of Atonement or Yom Kippur), could enter into it. If anyone else tried to go into the room at any time, or even if the High Priest entered it at any time other than to perform a special service on Atonement, they would be killed by God (Leviticus 16:1 - 2).
The Ark, which contained the words of the Covenant between God and his people, was very likely destroyed. It was likely burned when Nebuchadnezzar, king of Babylon, leveled Jerusalem (including the temple) in 586 B.C. We find an interesting statement in Jeremiah which declares that the ark and all that is with it will NOT be important during the millennium (Jeremiah 3:16). What will be important is the NEW covenant God offers all those who live.
It may well have been, shortly before or after the words of Jeremiah 3:16 were written, that this object representing God's covenant was destroyed by the Babylonian army. The book of Hebrews, chapter 9, explains that the spiritual or holy relics (if that is the right word to use) found in Jerusalem's Temple were patterns or copies of spiritual realities in heaven (Hebrews 9:1 - 5, 23 - 24).
Revelation 11 states that the Ark of the Covenant will be seen again in the near future. It will appear in the third heaven, the place where God's throne resides, after an angel loudly declares that God is coming back to earth to rule all humanity (Revelation 11:15, 19)! This ark is not the physical object once found in the temple and transported to heaven but rather is the original composed (likely) of spirit essence.