Europe's biggest infrastructure project is halfway done.

London's $25 billion public transit project (Europe's largest) is now halfway done. Crossrail, a 73-mile addition to the city's rail network, will connect Heathrow airport to Essex, adding an estimated 10 percent in ridership capacity.

Witnessing workers and machines build massive tunnels 80 feet underground since 2009 has been an interesting sight on its own. But the project also ended up being quite the archeological opportunity. An incredible range of remains -- from an ancient Roman road to graves of Black Death victims -- have been unearthed. 

Construction has proceeded with few hiccups so far, and the mega-transportation project still on track for a 2018 completion. Below, peek at the progress:

Work on the new Crossrail and London Underground station at Tottenham Court Road continues as traffic moves down Oxford Street in central London September 30, 2013. (REUTERS/Andrew Winning)
Britain's Prime Minister David Cameron, fourth left, and Mayor of London Boris Johnson, third right, visit a Crossrail construction site underneath Tottenham Court Road in central London, on Thursday Jan. 16, 2014. (AP Photo/Ben Stansall, Pool)
A worker uses a blow torch on part of a conveyor at Crossrail's Stepney site in east London September 25, 2013. (REUTERS/Andrew Winning)
Workers stand in the mouth of one of the tunnels at Crossrail's Limmo Peninsula site in east London December 14, 2012. (REUTERS/Andrew Winning)
An archaeologist digs out skeletons from the site of the graveyard of the Bethlehem, or Bedlam, hospital next to Liverpool Street Station in the City of London August 7, 2013. (REUTERS/Andrew Winning)
An archaeologist digs out a skull from the site of the graveyard of the Bethlehem, or Bedlam, hospital next to Liverpool Street Station in the City of London, August 7, 2013. (REUTERS/Andrew Winning)
A member of the the archaeological team from the Museum of London points to the present day position of London's Liverpool Street Station on a 16th century map of the city, Wednesday, Aug. 7, 2013 photo. (AP Photo/Alastair Grant)
Workers line up rails for the tunnelling machine at Crossrail's Stepney site in east London September 25, 2013. (REUTERS/Andrew Winning)
A worker rides a train car taking him out of the Crossrail tunnel being built from Paddington towards Farringdon under central London March 13, 2013. (REUTERS/Andrew Winning)
A worker surveys a tunnel entrance at Crossrail's Stepney site in east London September 25, 2013. (REUTERS/Andrew Winning)
A technician sprays concrete to support caverns built to house the converging railway tunnels at Crossrail's Stepney site in east London December 14, 2012. (REUTERS/Andrew Winning)
A worker stands on the tunnel boring machine creating the Crossrail tunnel being built from Paddington towards Farringdon under central London March 13, 2013. (REUTERS/Andrew Winning)
A worker emerges after the tunnelling machine has made the breakthrough into the station structure at Canary Wharf, in east London June 11, 2013. (REUTERS/Andrew Winning)
Concrete shatters as a tunnelling machine makes the breakthrough into the station structure at Canary Wharf, in east London June 11, 2013. (REUTERS/Andrew Winning)
Workers look on after a tunnelling machine made the breakthrough into the station structure at Canary Wharf, in east London June 11, 2013. (REUTERS/Andrew Winning)
A diver prepares to enter the water to work on support structures at Crossrail's Albert Dock site in east London March 27, 2013. (REUTERS/Andrew Winning)
A member of the crew of a bulk freighter prepares his ship to receive tons of earth generated by the construction of Crossrail, at a jetty on the Thames in east London, December 17, 2013. (REUTERS/Andrew Winning)

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