A sluice gate for a channel controlling the flow of seawater into a fish tank on Ventotene Island, south of Rome, with a modern-day red and white measuring rod. The gate is made of limestone with holes to allow flow during high tide, and it can slide vertically into the stone blocks to the left and right. When it was operating, low tide occurred at the level of the base of the sliding block. Today the entire gate is below sea level; rise in sea level since the time of construction in the second century A.D. is estimated at 1.50 meters (plus or minus 0.22).