A Mobile Underground Detention and Detainment center (or MUDD) was a secret prison network that held foreign nationals wanted for international crimes. Their existence was not officially recognized by the United States government.
As of Day 1, there were twenty-seven Level 3 Detention Centers across the Continental United States, all of which were formerly Strategic Air Command anti-aircraft sites. To ensure maximum security, prisoners were periodically transferred from site to site. Eleven prisoners were being held in MUDDs, including Liam O'Shea of the Irish Republican Army, Abdullah Ahmed Al-Adel, wanted in Israel, and Ichiro Nakada, who was wanted for involvement in the sarin gas attack on the Tokyo subway.