Between the branches of the trees, we can see two buildings of the University of Pisa,built in 1908, which house the institute of Physiology and Biomedicine (access from Via San Zeno). The construction work revealed the wedge-shaped foundations of a colossal structure with a circular or semi-circular plan from the Roman period. In all likelihood it was a theatre, or an amphitheatre. The area is referred to as petriccio, or a le grotte (the caves), an early medieval terms that can indicate a large open pit quarry. Thanks to the Decreta Pisana, inscriptions of 2 and 4 AD preserved in the Campo Santo, we know of the existence of a theatre, but there is no mention of the arena. The only clue is the position, in Roman times, of the bend of the Auser river, which marked the northern urban boundary of the city. We know that amphitheatres were generally built outside the settlements to isolate them, given the bloody characteristic of the shows and fights they often hosted. Unfortunately at the moment it is not possible to ascertain whether this was a theatre, or an amphitheatre.