The lower level of the Cathedral Basilica is home to the Mosaic Museum, containing displays that show how mosaics are designed and applied. The museum also contains the original Kilgen organ console, the throne used during the visit of Pope John Paul II and a collection of historic vestments and precious objects used in the various rites of the Church. There is also a crypt where the remains of Saint Louis’s Cardinals and Archbishops are interred. The needlepoint prie-dieux in the crypt was created by members of the parish for the use of Pope John Paul II during his 1999 visit to Saint Louis and the Cathedral Basilica.

The museum is open from 10:00 AM - 4:00 PM Monday through Saturday and 12:00 noon - 4:00 PM on Sundays. A contribution of $2 per person is requested for admission.