The Holy of Holies, as its name implies, was the most sacred part of the entire Temple. Entry was forbidden except on Yom Kippur when the High Priest entered the Inner Sanctuary.
Its dimensions were 20 cubits long, 20 cubits wide, and 40 cubits high. The floor, walls, and ceiling were plated with gold, as was the Kodesh.
In the Holy of Holies was a rock that projected above the ground to a height of 3 fingers. The rock was first uncovered by King David and the prophet Samuel.
Some say the rock was in the very center of the Holy of Holies; others say it was near the western wall of the Holy of Holies, and still others maintain that it was near the curtains separating the Kodesh and the Holy of Holies.
The Ark (which contained the tablets with the Ten Commandments) rested upon the rock during the First Temple era. During the Second Temple there was no Ark. Some say it was captured by Nebuchadnezzar; others say it was hidden beneath the rock; and still others say that it was hidden below the Wood Chamber in the Women's Courtyard.
There was another floor level above the Kodesh and Holy of Holies whose dimensions and decorative work were exactly like the chambers below, but with no curtains separating the floor above the Kodesh from from the floor above the Holy of Holies. Instead stone blocks projected from the wall itself to mark the division between the two sacred areas.
The floor of the chamber above the Holy of Holies had trapdoors surrounding the walls of the room. A large box supported by a rope, could be lowered through any trapdoor down into the Holy of Holies. Workers who had to repair the walls of the Holy of Holies were lowered through the trapdoors into the chamber below. Their view of the Holy of Holies was completely blocked off except for the portion of the wall to be repaired.