Britain’s oldest bath overflow is to be given its first thorough inspection nearly 2,000 years after it was built.
For two millennia the Great Drain has carried the mineral-rich waters of Britain’s only hot spring from the Roman Bath in Bath to the nearby River Avon. The drain runs for nearly half a mile under the city but although parts of it are large enough for a man to walk through, it has never been fully explored.
Archaeologists will have their first opportunity to get inside the previously inaccessible sections of the Great Drain this month when engineers open it up for repairs.
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A stretch of drain built long after the Romans is causing the difficulties. The extension was added when marshes were reclaimed from the Avon beyond the old city wall. It started to back up 18 months ago, causing a flooding risk in the city.
The Roman structure has easily outlasted the work of more modern engineers. A final section dating from the Sixties collapsed two years ago and had to be rebuilt.
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Miles Barnes, of Bath council, said: “The Roman engineers really knew what they were doing. Most of the drain is in absolutely tip-top condition and still doing the job it was designed for.”
When the Romans transformed the hot springs, which were sacred to the native Celts, into a religious and cultural centre, they built the Great Drain to prevent flooding in central Bath. It carries away the overflow after the hot water, which emerges from the springs at a million litres a day, has filled the baths.
When the site of the Roman Baths was originally excavated in the late 19th century, finds made in the Great Drain included 33 carved cameo gemstones and a mysterious tin mask.
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Mr Barnes said: “Gems were as rare and precious then as they are now. We don’t know whether they were put in the sacred spring as an offering or just dropped by accident.
“Although the drain is pretty clear there are sections where it bends or narrows where sediment will have built up that has not been disturbed for 2,000 years, so who knows what else we will find.”
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Before the engineers are allowed in, the drain will be explored by teams of potholers to ensure that it is safe.
When it was built the drain was only just below the level of the Roman streets. The original engineers installed square inspection hatches to allow it to be swept clean of sediment. After centuries of building in the city, the Great Drain is today at least 4m below ground level.
Archaeologist hope to discover what else the Romans dropped down the plughole before the project is completed next month.