Subway construction inherently concerns archaeologists as it can destruct valuable remains from history. But underground construction can also be the best opportunity for scientists to make discoveries and do excavations. And after construction, the subway stations can become very special places when they incorporate original artifacts.
Scientist's Brush versus TBM
Subway construction is expensive and involves much digging, as does archaeology. So at best, both go hand in hand. In archaeologically sensitive areas, archaeologists have two concerns: Firstly, preventing the accidental destruction of valuable information and objects from the past, and secondly, utilizing the opportunity to do research and excavation while the ground is opened up anyways.
Time is money, so the agendas of archaeologists and subway builders are often contradictory. Subway builders want to proceed quickly, while archaeological examination, manual digging, careful conservation and documentation can be extremely time consuming. Subway construction involves lots of staff, time-sensitive procedures like ground water pumping, or the use of expensive tunnel boring machines (TBMs). Someone has to decide whether a find has to be be removed, destroyed or encased within the subway's structures.
Unexpected discoveries can cause delays and huge impacts on building costs when construction plans have to be reconsidered. To prevent this, subway tunnels are sometimes built deeply underneath the historical layers. Nevertheless, lots of archaeological objects have been unearthed in many cities around the world during subway construction. Here's a list of examples.
Syntagma station (opened in 2000): The large upper concourse hall with its lots of archaeological displays.
[Photo by F. H. Langhorst, nitle.org]: Pino Suarez station: Aztec temple for the wind god Ehecatl. Daylight is coming in from above.
Reports from Different Cities
Athens: When two new metro lines were planned to be opened on occasion of the Olympic Games in 2004, this arose much concern in the international archaeological community since tunnels, ventilation shafts, and five stations located in the heart of the ancient city could destroy valuable information about the city's history. Athens has strict preservation laws, and the construction turned out to be an archaeological milestone. 30,000 artifacts were found on 70,000 square meters, the largest excavation project in Greece so far. The Ministry of Culture now features the most important of the excavated objects in permanent displays at three metro stations in the city centre (Syntagma, Evangelismos, and Academia) and at the University of Athens, Zographou campus .
Cologne: Right in the city centre, the construction of a 4 km long North-South light-rail tunnel between 2004 and 2010 means digging in an area of 30 km2. This spawned the biggest archaeological project ever in Cologne , while construction costs skyrocketed. Science determines the pace of construction here, and a team of up to 100 scientists and technicians has unearthed many archaeological artifacts from the city's Roman history, including a 2000 year-old wreck of a Roman freight ship and the remains of Roman wharfs .
Istanbul: Once known as Constantinople, Istanbul has many layers of cultural history below. Four archaeological sites lie in the path of the subway line currently under construction. Tunnels between the stations will be 30 m deep to run below the historical ruins, but shafts for escalators and concourses will penetrate the archaeological layer. A shipwreck has been discovered at Yenikapi, which was a harbor during Byzantine times. The pieces of the boat, which dates to either the 11th or 13th century, are being moved and eventually will be displayed as an exhibit in the station .
London: The Jubilee Line Extension Project has cooperated from the early planning stages in 1992 with the Museum of London Archaeology. The successful cooperation helped to prevent unplanned delays and lead to valuable discoveries about London's history .
Los Angeles: Subway construction aroused curiosity, not of archaeologists but palaeontologists, when a treasure trove of fossils was unearthed between Wilshire/Vermont and North Hollywood stations. The objects are not displayed in stations, though. About 2000 fossils could be preserved, dating back to up to 16.5 million years ago, including many fish species new to science, as well as bones and teeth of mammoth, mastodon, camel, bison and ground sloth as well as parts of redwood trees and other plants [6, 7, 8].
Madrid: The remodelling of stations led to the discovery of major archaeological and palaeontological findings, now turned into museums and open to the public, like the Caños del Peral Museum at Ópera or the paleontological site at Carpetana station. 
Mexico City: Skeletal remains of Pleistocene mammoths have been found in many metro construction sites. During construction, thousands of objects from early human settlements were uncovered. At Pino Suárez station, an entire Aztec pyramid sits in the passageway between lines 1 and 2. Many of the objects are displayed at the National Museum of Anthropology . On line 8, an entire Aztec neighborhood and a colonial-era Spanish hospital dating back to the 16th century have been found. The finds delayed subway construction and caused disputes over urban priorities .
Naples: Metro line 1 extension parallel to the coast leads along the ancient city wall through archaeologically interesting territory. Excavated objects are displayed in Museo station. In the future, more stations with archaeological exhibits will follow, including Duomo and Municipio stations (both to be opened in 2011) [9, 15].
New York: During construction of a replacement for South Ferry station, the project hit a 15-meters-long section of a stone wall that archaeologists believe is a unique remnant of the original battery. It has probably been built in the late 17th century and protected the Colonial settlement at the southern tip of Manhattan. The find may delay subway construction and will possibly be displayed in a park or museum .
Paris: During construction of the new driverless line 14 in 1990, several canoe-shaped boats have been found and excavated ten meters below the banks of the Seine river. The boats were made of hollowed-out logs and date back to 2800-2500 BC, making them among the earliest signs of human settlement of tribes in the area. They are now housed in the Carnavalet museum .
Parma: In this Italian metro under construction, the tunnels will run below the level at which archaeological findings usually are made. It has been decided to display in the future stations any artifacts located during construction of the shafts .
Rome: The relationship between planners and preservationists is difficult due to Italy's strict preservation laws and the fact that most of central Rome is of historical significance. Without paved roads, early cities used to rise gradually on the build-up of dust, waste, and horse excrements. Rome has risen about 15-20 meters over the past 2000 years, thus preserving many remains. Until now, only two metro lines serve the 2.5 million residents, leaving the city's streets regularly clogged with motorized traffic whose pollution in turn endangers historical monuments. A few years ago, planners and preservationists decided to work together on the new subway line C, to be opened around 2014. Tunnel boring machines will be used more than 30 meters below the surface, which is below the archaeological remains. But the surface has still to be opened up at places for constructing stations and ventilation shafts. As museum space is limited in Rome, many of the objects to be excavated are destined to be displayed in the stations [10, 14, 26].
Sofia: In the capital of Bulgaria, ruins of an old fortress and city wall have been discovered during the planning stage for the proposed lines 1 and 2. Two scenarios have been considered when planning the intersection beween those lines at Serdika station. The first option was to build the metro tunnels away from the archaeological site and to keep the archaeological research for later; the second option was to build the intersection beneath the archeological reserve, utilize the construction for excavation and set up a pedestrian underpass with an archaeological exposition. In cooperation with the Committee for Culture the second option has been chosen. [11, 15].
Thessaloniki: The second metro in Greece, which is still under construction, spawned Thessaloniki's largest-ever excavation site and will shed light on the history and topography of Hellenistic, Byzantine, and Roman periods. A memorandum of cooperation between the Ministry of Culture, Attiko Metro, and the construction firm specifies ways for how findings should be preserved, stored, protected, transported, restored, documented and displayed at the six underground stations along the ancient Via Egnatia .
Vienna: The capital of Austria began metro construction in 1969, an opportunity already being eagerly awaited by historians. Archaeologists have been involved from the very beginning of the construction work and could sometimes even helpfully warn the engineers about obstacles in the soil. The efficient cooperation resulted in lots of important archaeological findings while unexpected delays could be minimized. A subset of the artifacts is now displayed in a few stations .
While constructing Municipio station for the extension of metro line 1, remains of Roman harbour facilities have been found and are now subject of archaeological scrutiny.
[image of unknown origin, skyscrapercity.com]: model of Duomo station (to be opened in 2008): A Roman temple will be part of the station, visible in the upper left of the model.
Opera station: The station houses the Caños del Peral archaeological museum (230 square metres), featuring audiovisual displays, a 16th century fountain and other water supply works discovered during the station remodelling works in 2008, such as the “Arenal Sewer”.
Carpetana station: This is a significant site for paleontological remains in the suburban region. When installing elevators at the station in 2008, experts found over 5,000 animal remains of a wide range of genuses, dating from the Middle Miocene period (about 14 million years ago). Replicas of some of the fossils were installed, thus converting the station into a downright museum, including explanatory boards and showcases with replicas of the flora and fauna that once inhabited the place.
Repubblica station: Remnants of a Roman city wall throughout the station, partly behind glass.
Amba Aradam-Ipponio station on Line C station: To be opened around 2020: Rome is going to build "the world's first archaeological underground station" around ancient Roman barracks which came to light during the works.
Schwedenplatz station: Coats of arms that decorated the city wall in the Renaissance era.
Rochusgasse station: Exhibits showing finds from the Roman age.
Stubentor station: Remnants of the old city wall.
Stephansplatz station: The Virgil chapel, dating back to 1250, was excavated under the Stephansdom cathedral during metro construction. It can be seen through windows from the station's concourse level.
[Photo by untergrundbahnen.de]: Monastiraki station (opened in the year 2003): A bridge made of glass leads through ancient sewers and foundations.
[Photo by Willy Kaemena, mac.com]: Campo 24 de Agosto station: A fountain from the 16th century (Arca de Água de Mijavelhas) permanently exhibited in the concourse level.
[Photo by wien-diashows.com]: Stephansplatz station: Virgil chapel from 1250 seen through window in station concourse.
 (Athens) Parlama, Liana; Nicholas Stampolidis: Athens: The City Beneath the City: Antiquities from the Metropolitan Railway Excavations. Harry N. Abrams, 2001.