Yoma, Chapter Five, Mishnah Two
This mishnah teaches where the high priest would put the coal-pan in the Second Temple period when there was no longer an Ark.
With regard to the Ark, we have already learned in Shekalim 6:1-2 that there was a tradition that it was buried somewhere underneath the Temple floor. Other sages hold that the Ark was taken to Babylonian during the first exile in 586 B.C.E.
After the Ark had been taken away, there was a stone from the days of the earlier prophets, called shtiyah, three fingers above the ground, on which he would place [the pan of burning coals].
When the Ark was removed, there was a stone in the Holy of Holies that was there from the time of the early prophets. According to the Talmud, this stone was there from the time of Samuel and David, before the Temple was even built. Shtiyah is usually interpreted as the foundation stone for according to Jewish understanding, the world was founded from this point, either physically or spiritually. The stone was only three fingers high off the ground, but this was sufficient in order to put the coal-pan on it and then turn the incense into smoke.