What were the Inquisition's secret jails called?

The Inquisition’s secret jails were called Cárceles secretas.

The name referred chiefly to their secrecy: prisoners were taken away without their whereabouts being disclosed. Detainees could remain isolated for weeks or months, often without knowing the charge, the evidence, or the identities of witnesses against them.

These prisons were attached to inquisitorial tribunals across the Spanish monarchy. Their conditions varied considerably. Some were reportedly better than ordinary jails, while others—such as those at Llerena and Logroño—were notorious for darkness, poor care, illness, and deaths.

A common mix-up is treating “secret” as a description of hidden underground dungeons. The phrase primarily described secrecy surrounding detention and proceedings, not a single standardized prison design. Inquisitorial secrecy also extended beyond imprisonment: defendants were generally expected not to reveal what they had experienced after release.