The experienced team from St Albans District Council dug up part of a second century manor house buried under the park expecting to find a colourful mosaic tiled floor, discovered by Sir Mortimer Wheeler in the 1930s.
But instead they were greeted with remains of the room's foundations.
Simon West, who heads the team, believes Sir Mortimer could have taken the ancient pieces and sold them on for a penny each in the 1930s.
Oops.