Karen recounts how surgeons at this notorious prison for worst offenders were suspected of conducting experiments on the newly departed. Because of the prevailing belief in resurrection at the time this was a highly unethical practice.
She turns our attention to the innocuous looking sheep's skull.
"One night I was telling a story when the skull moved. Then it moved again."
At this point I look round at the audience. Everyone is spellbound and deathly silent. You could have heard a pin drop in the basement room.
Suddenly, Karen places her hand on the skull and slides it over the stone slab. This creates a horrendous grating sound that reverberates around the room. A chorus of screams rings out and people spring backwards, recoiling from the sound.
"It's alright," Karen says nonchalantly.
"On that other night a very frightened mouse was hiding under the skull. It jumped on to a lady's coat and ran up into her hair. She screamed and her husband, being a strong supportive type, screamed too. Everyone ran for the door, which opens inwards. They all believed that they were trapped with a ghost in the room."
Prison reformers in England designed Port Arthur Penal Colony as "a machine for grinding rogues into honest men".
After it closed in 1877, most Tasmanians maintained a strict silence about their convict ancestry, but today it's often lauded as a badge of honour.
As we gather in the stark Separate Prison, Karen explains that spirits, or ghosts, are believed by many to be lost souls trapped in a location, unable to rest.
These spirits seem to be able to occupy the fabric of a building - the walls, floors and ceilings.
I sense the frigid isolation prison is particularly conducive to paranormal activity.
Capitalising on this tense atmosphere, Karen tells how during a tour a World War II veteran entered one of the cells and experienced an intense feeling of despair.
He was found slumped against the wall, hugging his knees against his chest.
"We found him almost unconscious against ..." Bang! A large man collapses and crashes to the floor. Everyone gasps and several cry out. He struggles back up and says prosaically,
"My knee just gave out."
"You scared the bejesus out of me," screams our guide.
"You're not on this tour to give me the heebie-jeebies. That's my job. Anyway, as I was saying, this veteran was slumped against the wall precisely where an infamous prisoner hanged himself."
Our guide has the final word.
"If you enjoyed the tour, my name's Karen. If you didn't enjoy it, my name's Trevor.
"By the way, I take no responsibility for how well you sleep tonight."
CHECKLIST
Getting there: Air New Zealand flies daily to Hobart from Auckland via Sydney and Melbourne. Port Arthur is a 90-minute drive southeast of Hobart.
Further information: See portarthur.org.au.
