A green sofa sits on top plush-piled carpet inside the library at the University of Ballarat's SMB campus.

There are a number of them dotted around the place, but this particular sofa is helping keep one of Ballarat's most intriguing secrets.

Today staff pushed it back to reveal a displaced section of carpet covering a trapdoor.

The Courier was given access to the dank and gloomy cells beneath what was the Old Ballarat Gaol.

The repurposed building now houses Ballarat's brightest minds. In 1880, it housed Ballarat's most callous criminals.

Most of the cells have now been "sealed off", according to staff – a nice way of saying the clay and bluestone walls have caved in.

Now, the only known access to the rooms which housed bank robbers, bush rangers and murderers is a small trap door beneath the sofa in the library.

There were fears the air inside the access shaft and the cells themselves would be stale and make the visit impossible.

But as soon as the hatch was lifted, a cool breeze blew from the hole in the floor, confirming hidden tunnels – somewhere – were supplying the long forgotten area with some fresh air.

Just to be on the safe side, experts accompanied The Courier down the hole and into those places of solitary confinement with an oxygen meter.

The readings said levels were slightly lower than the air on the outside, but more than enough for sometime to survive – and take pictures.

Inside, a stone barrel-vaulted roof a little over 2m tall joined run-down clay and bluestone walls.

Moisture could be seen seeping through the cracks in the bluestone blocks creating an eerie, damp environment.

During its time in use between 1864 and 1908, 13 executions took place at the Ballarat Gaol – the highest number anywhere in regional Victoria.