Archaeological investigations at Mytilene began in the late 19th century when Robert Koldewey (later excavator of Babylon) and a group of German colleagues spent many months on the island preparing plans of the visible remains at various ancient sites like Mytilene. Significant excavations, however, do not seem to have started until after the First World War when in the mid-1920s Evangelides uncovered much of the famous theatre (according to Plutarch it was the inspiration for Pompey's theatre in Rome in 55 BC, the first permanent stone theatre in Rome) on the hill on the western side of town. Subsequent work in the 1950s, 1960s and 1970s by various members of the Archaeological Service revealed more of the theatre, including a Roman conversion to a gladiatorial arena. Salvage excavations carried out by the Archaeological Service in many areas of the city have revealed sites going back to the Early Bronze Age although most have been much later (Hellenistic and Roman). Particularly significant is a large stoa over a hundred metres long recently dug on the North Harbour of the city. It is clear from various remains in different parts of the city that Mytilene was indeed laid out on a grid plan as the Roman architect Vitruvius had written.