The city is spending $142 million to renovate Gaillard Auditorium.
But now workers have discovered more than two dozen graves and have more areas to search. Officials are puzzled by the graves since there is no record of a cemetery at that location.
Archaeologist Eric Poplin says it appears the pits date to at least 1780, the time of the American Revolution.
Dustin Clemens with the city says it could take a couple of days to decide what should be done with the remains.
The graves are spread out in two even rows. Heavy machinery, shovel and trowel work removed tons of dirt in an area previously sealed under an auditorium parking lot.