When the boulevard was cut through in the 1970s, the excavation went straight into a late Roman townhouse with its mosaic floors intact. Rather than lift them, the city built the underpass over the top and left them where they were, and Trakart is what you reach by walking down into it. The mosaics are fourth-century, geometric and figurative, with birds and a well-known peacock, and you look at them from a walkway a metre above the floor level they were laid at.
The centre also holds a collection of Roman and Byzantine glass — small, unfussy, and worth the ten minutes. Labelling is in Bulgarian and English and the staff will usually walk you round if it is quiet, which it generally is.
The entrance is under the Tsar Boris III Obedinitel boulevard near the Roman Forum, and it is easy to walk past. Look for the stairs down rather than a building.