Modern İzmir is growing in several directions at the same time. The north-western corridor extending to Aliağa brings together both mass housing projects, including villa-type projects and intensive industrial area, including an oil refinery. In the southern corridor towards Gaziemir yet another important growth trend is observed, contributed to by the Aegean Free Zone, light industry, the airport and mass housing projects. The presence of the Tahtalı Dam, built to provide drinking water, and its protected zone did not check urban spread here, which has offshoots in cooperatives outside the metropolitan area as far south as the Ayrancılar-Torbalı axis. To the east and the north-east, urban development ends near the natural barriers constituted respectively by the Belkahve (Mount Nif) and Sabuncubeli (Mount Yamanlar-Mount Sipylus) passes. But the settlements both above Bornova, inside the metropolitan zone, and around Kemalpaşa and Ulucak, outside the metropolitan zone, see mass housing and secondary residences development. More recently, the metropolitan area displays growth, especially along the western corridor, encouraged by the Çeşme motorway and extending to districts outside the city of İzmir proper, such as Seferihisar and Urla.