A total of 18.2 km of tunnels and 18 stations have been built on line 1. However, a further 4 km of tunnels and 5 stations are also under construction. With a total cost of 3.8 billion Euro (270 million for the archaeological work alone) for line 1, and 790 million Euro for line 6 (54 million for archaeological expenses), this is the largest infrastructure project in the region of Campania. In 2017, an estimated 45 million passengers used line 1. This is the metro with three A’s (architecture, art and archaeology). It has an incredible cultural heritage, valued at 58 million Euro in artworks and architecture, with 45 million spent on urban redevelopment compared to the 1.4 billion spent to build the so-called lower section of line 1. The latter has also won numerous awards, with some stations cited as amongst the most beautiful in the world (Toledo and Municipio). Advanced techniques were used to allow excavation within the water table. This major engineering challenge resulted in the use of liquid nitrogen to freeze the ground when digging the tunnels. This, along with other innovative technologies implemented, makes it one of the most researched underground railways in the world.