On Thursday evening at Founders’ Hall, municipal surveyor Pella Strand laid forty-seven hand-drawn maps across three trestle tables and told the story of a buried river.

The audience — 280 seated, standing room exhausted, the corridor occupied by those who had arrived too late for the hall — listened for ninety minutes as Strand traced the route of the Lower Conduit from its stone-arched outfall beneath Bramblegate Steps to the cistern chamber beneath the old Ashwater Brewery, through the 1760s mason’s-mark section with its fish-in-a-circle guild stamp, and out along the western branch where the original spring still flows after 244 years.

“Nine hundred and forty-seven metres,” she said, tapping the final map. “Seventeen notebooks. Twelve cross-sections. And I still cannot find the spring source.”

The spring, she explained, runs somewhere beneath the brewery district, feeding the cistern that once supplied three Cooperage Lane breweries. The water enters the conduit system through the western terminus — a section where the brickwork narrows to a crawl space and the floor is submerged — and she has not yet been able to survey beyond it. “The conduit continues,” she said. “I simply cannot follow.”

The Heritage Committee received Strand’s 60-page report on 28 March. The report recommends heritage listing for the entire system — the first such designation for a subterranean structure in Bobington. A decision is expected within the fortnight.

Strand was asked about the connection between the conduit outfall and the seal colony on the mudflats below Bramblegate Steps. She noted that the 1891 annotations in Clement Birch’s Fauna of the Lower Ashwater, discovered at Fowler’s Books this month, describe a “freshwater spring beneath the wharf” at the precise location of the conduit outfall — independent historical confirmation, 135 years old, of the relationship between the waterway and the habitat.

“Birch saw the seals,” she said. “He saw the spring. He drew a map. Then the city built over it and forgot.”

Sylvia Hatch, the Polytechnic’s deputy librarian, has begun cataloguing the Birch volume. Dr Emmeline Furness, whose own lecture on the Meredith letters opened the series on 8 April, attended and was observed in conversation with Strand afterwards.

The ovation at the end lasted over a minute. Strand, who has spent much of the past three months underground, appeared briefly uncertain about what to do with her hands.

Miriam Aldbury confirmed that the closing lecture on 29 April remains unassigned, though Professor Nettleford has been invited to return.