Nine test trenches were excavated in the grounds of the Mercury Theatre in 1996, ahead of proposed extensions to the Theatre, with the aim of determining the depth at which the uppermost archaeological deposits survive. The nine trenches were excavated by machine, followed by manual cleaning of the upper archaeological layers and limited excavation of selected features to clarify the exposed stratigraphy.<1> Archaeological remains were encountered in all trenches. No post-Roman structural features were encountered although a gravel layer below the garden topsoil in Trench 1 was of unknown date. Evidence of Roman buildings was found in all nine trenches. A common feature of the uppermost archaeological layers was the presence of clay-rich deposits variously representing destruction debris and constructional make-up between different phases of building. A notable discovery in Trench 8 (to the SW of the Theatre), c.1m below ground level, was a mosaic floor. The mosaic is enclosed by a 1.1m wide border of red tesserae and features a black and white chequered pastern within a 0.57m wide decorative geometrical surround. Trench 9 also revealed a well preserved stretch of tessellated floor (100mm below the level of the mosaic in Trench 8) which, for reasons of distance, width and level, is thought unlikely to belong to the same room as the mosaic in Trench 3.