Yoma, Chapter Five, Mishnah 2. Our Mishnah continues the subject we examined in the previous Mishnah: the state of the Holy of Holies after the Ark was removed from it - whether it was taken into the Babylonian exile or hidden away by King Yoshiyahu in the vaults beneath the Beis HaMikdash.
Let us look at the words of the Mishnah:
"Mishenital ha'aron" - from the time the Ark was removed from its place in the Holy of Holies.
"Even haysah sham mimos nevi'im harishonim" - in the place of the Ark there was a stone, which had been there from the days of the earliest prophets, that is, from the days of Shmuel and David - even before the Beis HaMikdash was actually built.
"U'shesiyah haysah nikra'as" - this stone was called "shesiyah," meaning the foundation stone, and it was called so because the world was founded from that stone: it is the central point of creation.
"Gavo'ah min ha'aretz shalosh etzba'os" - the stone was raised three fingerbreadths above the ground.
"Ve'alehah hayah nosen" - upon that stone the Kohen Gadol would place the fire pan, in order to put the incense on the coals.
In summary: once the Ark was removed, the shesiyah stone served as the place for setting down the fire pan in the Holy of Holies. This stone predated the building of the Beis HaMikdash and stood there from the days of the earliest prophets, Shmuel and David. It was called "shesiyah" because from it the world was founded - the central point of creation - and it stood three fingerbreadths above the ground.