Written by By Ben Wedeman, CNN Rome

Thirty-three feet underground, at the bottom of a concrete-lined pit, archeologist Gilberto Pagani patiently scrapes dirt from a charred beam of wood that has laid undisturbed for around 1800 years.

It's part of a house, perhaps once belonging to a senior Roman army officer, destroyed by fire. Last year construction workers discovered the site near the Colosseum as they were digging a shaft to the tunnel of Rome's new metro Line C. It was only this week that archaeologists revealed what they've found during the ensuing excavation.

This discovery is particularly interesting because the fire that destroyed the house left some things intact, including wooden beams that, under normal circumstances, would have decayed ages ago.

"It's an extraordinary situation," says Rome's archaeological superintendent Francesco Prospetti. "The collapse of the ceiling sealed everything inside. It was carbonized without being burned."

The skeleton of a large dog was also found at the bottom of the pit, its jaw and large front teeth easily recognizable.

Rome metro dig unearths 1800-year-old ruins

According to archaeologist Simone Morretta, "This poor dog was already in the room during the fire. We found ashes under its paws. Probably part of the burning ceiling fell on it and there it was stuck and died."

Fires in ancient Rome, she says, were frequent.

"Roman houses were full of wooden elements," she says. "There were lit by fire and cooking was done over an open flame."

This is just the latest in a series of archaeological discoveries during Line C's construction, which started in 2007. Last spring an army barracks was discovered nearby, and in 2009 construction was delayed when work ran into the remains of Emperor Hadrian's Athenaeum, an elite school dating back to the second century.

Each discovery has caused construction delays, but Prospetti bristles at the suggestion progress is being sacrificed to the past.

"It was foreseen in the planning for Linea C that there would be plenty of time set aside for archaeology ... Our effort is to transform an apparent hindrance to public service into a great opportunity, by giving Rome a subway unique in the world, in which you go underground not just to take a train, but also to take a journey in history."