Ark of the Covenant
Once a year, on the Day of Atonement, the High Priest was allowed to enter the Holy of Holies.
And in the Holy of Holies was the Ark of the Covenant.
And they shall make an ark of shittim wood: two cubits and a half shall be its length, and a cubit and a half its breadth, and a cubit and a half its height. And thou shalt overlay it with pure gold, within and without shalt thou overlay it; and thou shalt make upon it a crown of gold round about. … And thou shalt make a cover of pure gold; two cubits and a half shall be its length, and a cubit and a half its breadth. And thou shalt make two cherubim of gold, of beaten work shalt thou make them, on the two ends of the cover. … And I will meet with thee there, and I will speak with thee from above the cover, from between the two cherubim which are upon the ark of the testimony, all that which I will command thee unto the children of Israel. (Exodus 25:10-22 [Leeser])