HMP Shrewsbury: Rare view of hidden Georgian prison
24 March 2013 Last updated at 15:43 GMT
The Victorian part of HMP Shrewsbury, also known as the Dana, was built directly on top of the Georgian original, according to Gerry Hendry, the jail's last governor.
The builders partially filled the Georgian holding cells with concrete to provide foundations for their new building, he said.
Mr Hendry said the trapdoor in the original room where prisoners were hanged can still be felt under the floor.
The last inmates were transferred from the category C prison to other prisons last month.