Steve Farrar donned his hard hat and joined a scientist who delves into the depths in search of a highly hazardous inhabitant.
We blunder into the pocket of gas by accident. The short side passage with its towering arch of glistening flowstone is a particularly pretty part of Clapham Cave. So we trudge off the main route through this popular Yorkshire tourist attraction to take some photographs.
It is only when Gavin Gillmore checks his handheld radiation detector minutes later that it becomes apparent we were not alone in that passageway.
We had inadvertently been standing amid a concentration of colourless, odourless radon. For some 15 minutes, we had been subjected to a silent bombardment of radioactive particles given off by the gas's natural decay.
Gillmore, senior lecturer in environmental science at Bradford University and a director of the Radon Council, the radon industry's non-profit making regulatory body, assures me there is nothing to worry about. He seems more pleased than concerned at the revelation. The peak reading, although higher than other readings gathered in previous days deep inside the cave, is still relatively low and poses little threat to our health.
Clapham Cave seems to have been regularly scoured of its radon by the flow of air through the passageways. It is of scientific interest only.
Other locations Gillmore has visited have proved rather different. The scientist's work has revealed some of the UK's most dangerous radioactive hotspots, hidden unnoticed beneath our feet. But not, he fears, unvisited.
It was not the exceptionally high level of radon that chilled Gillmore during a recent visit to an abandoned tin mine in Cornwall. What upset him was the discovery of empty champagne bottles close by. The subterranean revellers had been guzzling bubbly where levels of radioactivity far exceeded the safety guidelines. They were so high that Gillmore believes they could redden skin through radiation damage within an hour.
While the 3 million becquerels per m3 of radioactivity betrayed one of the most concentrated pockets of the gas ever recorded, Gillmore has seen worse. His highest published level stands at 7 million becquerels per m3 at Bushdown mine, also in Cornwall. Preliminary data from another location recently identified the most radon-rich location yet measured, with four times Bushdown's concentration.
Radon is created by the radioactive decay of uranium-238, which occurs in many rocks. Its great density causes it to accumulate in poorly ventilated, enclosed spaces - such as a cave, mine or the basement of a house.
It is inert but undergoes rapid radioactive decay, emitting alpha particles and other by-products as it does so. If radon is breathed in, these particles damage delicate lung tissue, disrupting normal cell function, which can lead to cancer.
Each year, radon in the home is thought to be responsible for the deaths of some 2,500 British people through lung cancer. There are suggestions that it is also linked to heightened incidence of myeloid leukaemia, cancer of the kidney and certain other cancers. To give some idea of the scale of the problem, the Department of Health estimates that 1,000 people die annually from passive smoking.
Gillmore's calculations, published in the journal Health Physics , suggest that at the highest levels of radon that he has identified underground, the radiation can affect skin, with the potential to cause deadly melanomas.
There are some 3,000 caves around the UK and large numbers of abandoned mines. Many provide an ideal resting place for radon gas emerging from the rocks.
While potholing and mine exploration are minority activities, thousands of people plumb the depths of the earth every year. A disproportionate number are students, usually members of university-based societies. They have some awareness of the radon problem but may still be exposing themselves to an unsuspected threat.
The National Caving Association, in consultation with the National Radiological Protection Board, produced a booklet on the risks associated with radon some eight years ago. It urges cavers to keep within the NRPB's recommended maximum exposure values by finding out about likely problems in particular caves and calculating the potential dose received during a year.
It notes: "The NCA recognises radon exposure as one of the potential underground hazards and will endeavour to ensure that members are kept updated on the nature and extent of the risks as new information becomes available."
Accordingly, the association carries out research into subterranean radon concentrations. Clark Friend, convener of the NCA radon working party, says experiments have been recently completed in Ogof Ffynnonn Ddu, South Wales, and Wet Sink in the Forest of Dean. Yet lack of resources prevents this surveying from being comprehensive.
"We take this work seriously, but equally you must be aware of the considerable number of caves and mines and the costs of carrying out each experiment," Friend says.
The risk has to be kept in proportion, according to Eddy Hill, secretary of the NCA. "Most caving trips are in continuous movement through a cave and hence cavers would be exposed to a specific location for only a short time, in the order of minutes," he says.
Gillmore appreciates the NCA's efforts but believes the impact on cavers may be underestimated. "In theory, the NCA is aware of the situation and gives guidance," he says. "In reality, many caving groups ignore it. A lot argue that they are not down there long enough for it to be an issue. But at very high levels, radon can still have an impact."
Many of the student cavers interviewed by The Times Higher admit that awareness of the problem has little impact on their activities. Natasha Durham, a politics undergraduate and president of the caving society at York University, is typical when she observes that the caves visited by the society's 15 to 20 active members have a strong flow of water and air through them and hence are unlikely to possess high levels of radon.
She adds that while society members are well drilled on safety issues, it is important to put all the risks in perspective. "There are so many scares that if you believe every one of them, you wouldn't do anything," she says.
But Gillmore believes this particular problem should be taken more seriously. "My concern is that there is a significant risk to people who go down caves and mines," he says.
Radon is not perceived as an especial threat even by homeowners who live in areas thought to have a high risk of radon accumulation.
Among cavers, the lack of information on concentrations in particular caves makes the risk difficult to evaluate. Some, such as those checked by the NCA, have been evaluated but most have not.
Cavers accept that their sport can be hazardous. They understand the risks of falling down a crevice or drowning in rising floodwater and take precautions. By comparison, the vague threat from radon can seem remote. As yet, no study has revealed any heightened incidence of lung cancer or melanoma in the caving community. Without a comprehensive survey of radon levels, all cavers can do is to avoid the hotspots identified.
Gillmore's work suggests these can be found in a variety of locations, including popular caves such as Swildon's Hole in the Mendips. He notes that high concentrations often occur in unexpected areas and points out that as radon levels depend on the movement of the air, humidity and temperature, significant variations can occur according to the time of year.
Despite our encounter with an isolated pocket of the gas, our visit to Clapham Cave in early spring revealed very low levels overall. But since then, Gillmore has plotted a rise - though still well within safety limits - as air flow responds to warmer weather outside.
Gillmore believes it is vital that the number of trips cavers make to radon-rich caves and mines be restricted. This will require more research, greater awareness of the problem and better discipline on the part of enthusiasts. He accepts that some feel this would infringe on their right to explore underground. But most will be happy to receive a friendly tip-off about whether this silent killer is lurking in the most innocuous side passage.