Rome, May 2 - A regional audit court has launched an investigation into the delayed construction of Rome's metro C line over concerns surrounding the foundation of the Colosseum and rising public costs, Italian heritage and environmental group Italia Nostra said Thursday. The probe comes "following our serious concerns announced April 12," said the group, citing the "chance that the stability of the Colosseum could be at risk, that construction costs could rise, and that the economy and tourism to the city could be damaged". Last year the national audit court said in a 182-page report that Rome's newest subway track appeared destined to become the slowest and most expensive construction project in Italian and European Union history, and risked not being finished. "All of the problems, vices and defects of public works in Italy are visible," said Audit Court President Luigi Giampaolino at the time. Building the C line, Rome's third underground line, has cost the city over five billion euros in public funds - three billion more than initially forecast when building began in 1990. It was originally slated for completion in 2000 when millions came to Rome to celebrate the Catholic Church's Year of Jubilee. A more recent deadline was 2020, when Rome was angling to host Summer Olympics. That bid was turned down by the government, and the audit court said that construction of the C line was still off pace to be completed in seven years. "It's not the fault of one single project or of the person who managed it," said Audit Court President Luigi Giampaolino last year. "This project includes every challenge imaginable - institutional, procedural and technical". Administrators point out that building a subway line in the ancient city is a particularly difficult due to the endless trove of artefacts underground that must be excavated before construction can proceed. They have also had to fight off allegations of waste and corruption within Rome's department of transportation, most recently in a probe into alleged kickbacks paid on city bus contracts. The extent of the risks - if any - facing the Colosseum is still unclear. The Colosseum, or Flavian Amphitheatre, is close to 2,000 years old and feeling its age as restoration work has been stalled for years and only recently cleared the last bit of red tape. It is 189 meters by 156 meters and covers an area of 24,000 square meters - putting it right at the entrance of a bustling metro stop. Capable of seating 50,000 spectators, it was used for gladiatorial contests, mock sea battles, executions and re-enactments of mythological dramas. Today, thousands of cars whiz around it during daily rush-hour traffic, creating a patina of black soot around its exterior. Italia Nostra and other activists are pushing to make the entire historic center accessible to pedestrians only.