DAVID Walker and Julia Clarke were in the caves beneath the Paul Smith shop in Low Pavement, setting up a fairly expensive laser.
That activity has a slightly sinister sound, like maybe Sir Paul has turned Bond villain and is preparing to release a secret weapon on an unsuspecting world.
But David and Julia work for the University of Nottingham, and their laser is not of the city-levelling variety. It helps them create digital moving images of the cave, which will soon go up on the internet alongside other images that take viewers on realistic flythroughs of many of the caves under the city.
The videos are part of the Nottingham Caves Survey, the two-and-a-half-year project David, an archaeologist for the university and Trent and Peak Archaeology, leads.
"We're about halfway through the project as it stands at the moment," David said.
"If we can survey 100 caves to the standard you see on the website, that would be a good dent and a good advertisement for Nottingham's caves."
They recently surveyed cave number 50.
"That now takes us to a grand total of about 10 percent of Nottingham's known caves," Mr Walker said.
The project was never meant to survey every cave under the city. Instead, it's meant to be a baseline study, a point from which future researchers can study and build.
But they're still trying to discover as many new caves as possible. To that end, they've put the call out for people who might be living or working on top of caves. People have got in touch.
"We're getting better known, which is good," Mr Walker said.
"We're still finding out about new caves that have never been recorded before ... or ones that have been just sort of forgotten about."
There's the Peel Street caves under the Arboretum – a huge underground labyrinth of sand mines.
There's the one in Fishpond Drive, The Park. Set behind a block of 1970s flats, it's a Victorian folly or summer house set into rock. It would have been an up-market Victorian plaything. Figures of Samson and Moses are carved into a bench.
"It's a seriously ornate bit of work," David said.
Other caves, like the ones under Willoughby House, have a more utilitarian, functional look.
One of those was a cellar cave, another was converted during the war to an air raid shelter. "It still retains some of its air raid shelter paraphernalia," David explained.
"And it still has an air raid toilet. Everybody loves a toilet."
They enjoy finding that "time capsule stuff". But Victorian follies or Second World War air raid shelters aren't quite the holy grail.
"The real goal will be to find a cave that we can definitively date as being Anglo-Saxon," he said.
"But there are no caves surviving that anybody knows about."
About 10 percent of catalogued caves are demonstrably medieval.
"I guess there are probably more Victorian ones that anything else," David said. "A lot of the standing buildings in Nottingham are from that great Victorian expansion."
For many Victorian pubs and even houses, something like a cave larder was quite popular.
"As well we see in the rich folk of The Park particularly ... cutting caves for wine cellars, and for show," he said.
"People are using their environment and showing them off one way or another."
Over the years, the showing off has often stopped. Some were walled off and forgotten about. Others – ones that have more public entrances – had different problems.
They've housed the homeless, been used by drug users and generally fallen into as much a state of disrepair as a cave can fall into.
"This has been one of the problems of the caves, as it were," David said.
"They have been increasingly locked up and blocked off over time."
He would like more made of promoting the caves – perhaps with leaflets and maps.
He likes "day caves" and "night caves" – public caves that can be toured by day, and caves under pubs that can be visited at night.
Publicans and other businesspeople who have caves might be more inclined to make the not insubstantial investment of bringing them up to modern safety standards, if they thought people were going to visit.
But that's a debate for another day.
"We're still really at the early stages of pushing that end of it, pushing what comes next," David said. "But we're doing quite well at the survey part of it."
The main goal is to catalogue and help preserve the caves. But David doesn't just want to see them turned into stagnant museum pieces.
"They've been dynamic for the last 1000 years. They've changed over time," he said.
"There's no reason why they can't be used in a more interesting way."
To learn more about the Nottingham Caves Survey and see the cave videos, visit www.nottinghamcavessurvey.org.uk.