For those who do not know (ha, me included!), it's a hypocaust flue, and to quote the accompanying sign:... "with part of its tile cover still in position, through which hot air was intended to reach the central chamber before being drawn through the diagonal ducts and up box flues in the walls."  This floor was never completed due to a disastrous fire which immolated part of the palace in the late 3rd century --- it was after this fire that a decision was apparently made not to rebuild the palace.