Very occasionally, the public are allowed rare access to the only remaining bits of Clerkenwell House of Detention, also known as the Middlesex House of Detention. They are the underground vaults, here pictured.
This institution was primarily a remand centre for prisoners awaiting trial at the nearby Middlesex Sessions House (still standing in largely original condition).
The main incident for which this prison is well-known — if at all — was a Fenian bomb attack on 13 December 1867. It became known as the Clerkenwell Outrage. The plan of this attack was to break free Fenian prisoners, primarily one Michael Blake. In addition to killing 12 bystanders, the attack was a fairly disastrous affair, no prisoners escaped. The Fenians weren't exactly criminal masterminds, they borrowed matches for the explosion from some passing children.
Both prisons were demolished in the years ahead, Clerkenwell in 1890 and Newgate in 1907.