Abstract: A watertight manhole insert and tether apparatus designed to prevent rainwater from entering sanitary sewer or other systems through a manhole closed by a manhole cover. The manhole insert is located beneath the manhole cover and is connected directly to the manhole by a tether apparatus mounted in the manhole, to plug the manhole, prevent theft of the manhole insert and deny access to the manhole. The manhole insert is characterized by a pan-shaped insert body having a curved, flat or ribbed insert bottom and one or more pressure-relief valves, one or more lift straps and an upward-standing, sloping or perpendicular side wall fitted with an outwardly-extending top rim for engaging a shoulder shaped in the manhole to support the manhole insert in the manhole.