Archaeologists say remains of a rare, circular 1850s prison block unearthed at the former Pentridge Prison is of world significance in penal history.

The public will next month be able to view the extraordinary bluestone foundations of the panopticon, shaped like a Trivial Pursuit token, which experts say is one of the few examples of its type to survive.

It was part of a brutal 19th-century movement to keep prisoners in solitary contemplation, under total surveillance.

Pentridge had three: this one, next to A Division on the north of the site is the first to be unearthed. Later in May, excavation will start on two more next to B Division to the south.

Archaeologist and ABC TV host Adam Ford said a teenage Ned Kelly, when at Pentridge for three years in the early 1870s, would have spent time in at least one of them. ‘‘He went in to Pentridge a 16-year-old as a ratbag and came out as a hardened criminal with a big chip on his shoulder.’’