In 1971, archaeologist Patrick Ottaway discovered Roman sewers beneath the cobbled streets of York. Dan meets up with Patrick for a journey back in time; going underground to take a closer look.
Crawling down narrow tunnels may not be comfortable for a strapping man like Dan, but this was the least of his worries - Dan was more concerned about what exactly he was crawling through.
"There was Roman poo here when this was originally found 35 years ago. But I think this is all gone now, more or less," Patrick tells a relieved Dan.
Only about 40 meters of the sewer has been excavated, but it's clear to Dan that they are an impressive feat of Roman engineering. At the time, the Romans made sure they took care of them - sending slaves down with shovels to clean them out. Now, after nearly two millennia, they are still in very good condition.
But what Roman buildings did this sewer serve? One possibility is just around the corner... Dan meets archaeologist Katherine Bearcock at the Roman Bath Pub who shows him the remains of the bath house that still exists in the pub cellars.
Every day, as the Romans bathed, they would go through about 70,000 gallons of water - all of which had to go somewhere.
There is evidence to suggest that the waste water did go into the sewer - items of Roman jewellery, including a pendant, have been found inside the sewer and Katherine thinks they may have been washed down after being lost in these baths. It's a good theory but so far, no one's been able to prove it.
Dan hopes this might change. Anthony Masinton, from York University, is using ground-breaking radar technology to see if he can establish a link between the baths and the sewer.
He is looking for secondary shafts, or offshoots, which link the sewers to the bath house. In order to find them they have to search the ground below a row of shops, which makes for a strange request of the local shopkeepers.
But alas, the radar can't see the missing Roman shaft through all the different layers of history. "There's a lot of stuff covering up the Roman layer. You've got the Anglo-Saxon layers, the Viking layers, the medieval layers and the Georgian and Victorian layers all on top of that, which masks something down at the bottom," Anthony explains to Dan.
Despite the disappointment at the radar's failure to detect any secondary tunnels, the sewer is still an incredible achievement. The Romans left Britain in 400AD and it was another 1,400 years before the British started building proper sewers themselves.
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