Midtown Manhattan will soon be upended by 20 years of construction as the Metropolitan Transportation Authority begins repairs to an underground entryway to Grand Central Terminal.

The state-controlled transit authority needs to overhaul a series of bridges and a viaduct that form a 75-acre train shed housing the terminal’s platforms and tracks.

The 1.8-mile viaduct is better known to New Yorkers as Park Avenue. The bridges are its adjoining sidestreets running from East 45th Street to East 57th Street.

MTA officials say it is too early to say how much disruption the work, which begins in May, will have on car and foot traffic. But beginning in 2021, the authority will start taking lanes and sidewalk from Park Avenue and its cross streets to conduct the repairs.

The MTA’s total budget for the project, excluding inflation, is $2 billion.