It began as a mission to map one of the world's most complex and intriguing subterranean cave networks.
Described as "the Great Barrier Reef of caving", the interwoven network of caves and sumps in Mexico's Cueva Alpazat extends more than 60 miles.
But six British servicemen attempting to unravel its mysteries were trapped last night after rising underground waters blocked their passage to the surface.
The Britons - five from the services and one civilian - have been holed up in the network of caves, about 110 miles (175km) northeast of the capital, Mexico City, since last Wednesday.
Two expert divers from the British Army were last night flown out by the Ministry of Defence (MoD) to begin a risky operation to rescue the men from the flooded cave.
The potentially dangerous operation would end a five-day ordeal for the men, who went down into the caves in an attempt to map out one of the world's great unknown underground cave systems.
The divers were expected to arrive last night so that they could assess the risks involved in bringing up the team using underwater breathing equipment.
The vast network of underground caves is notorious for filling up rapidly with water when flash floods occur above the ground - especially in spring.
And not long after the team from the Combined Services Caving Association (CSCA) entered the caves late last week, that was precisely what happened. "They were caught by unforecast rain," said a spokesman for the Ministry of Defence.
But the CSCA's "Cuetzalan Tiger" expedition had not gone unprepared. Like any experienced caving team, they assessed the risk - and took five days' worth of food, sleeping bags, radios and even novels.
They also knew that a previous expedition had stashed another five days' worth of supplies in parts of the cave system that never flood. They also had a "mole phone", which uses magnetic currents in the ground to communicate.
The problem now, though, is how - and when - they can get out. For the rains have shown no sign of stopping. And the flood waters are not receding.
It's not the first time that a Forces' expedition has run into trouble. In 1994 an Army expedition to Borneo, attempting to make the first descent through Low's Gully on Mt Kinabulu, got lost and broke the cardinal rule of such expeditions by splitting up. Half of the group managed to crawl out, but it was several days before the other five - two officers and three civilians - were able to gain their freedom.
It is hoped that this expedition will not suffer such a dramatic fate. The team of six - five from the CSCA and a British civilian who is acting as a guide - have been holed up above a sump, with wetsuits to keep them warm and rations.
Nor is finding water - usually the heaviest item to carry on any field trip - a problem. "I think their problem is having too much fresh water," quipped Mick Day, chairman of the National Caving Association.
Mr Day noted that the limestone caves are subject to frequent flooding: "When that happens this particular cave has an entrance which can flood for a length of up to 100 metres - it fills up to the roof. You can't get in or out."
Unless, that is, you stop waiting and enlist a cave diver.
"They would swim down with a smaller version of Scuba-style gear, with smaller tanks and a rope which would be used like a handrail," explained a spokesman for the Ministry of Defence. Then all the the trapped cavers would have to do is don the breathing apparatus and swim up through the sump to the exit.
A MoD spokesman said: "At the moment we think that the sump has receded to around 1.5m of water. The thing is that if you haven't done it before, then it could be slightly scary, like your first Scuba dive."
Jose Ignacio Macias, spokesman for the Civil Protection Agency in Puebla state, where the cave is located, insisted that the currents were "what stopped them from getting out". He added "perhaps they had no rope to get out, but for the moment we don't know much more about their situation."
The MoD - perhaps mindful nowadays of avoiding any repetition of the Low's Gully fiasco - was dismissive of the idea. "A cave collapse is not part of the situation report we have received from the field officer in the location," said the spokesman. "There are some errors in what's being reported because some over-excited local officials are talking to anybody. The British officer there says they're going to sit it out for now."
But that decision has clearly irked some of the would-be Mexican rescuers, who seem to be surprised that a party whose members have explored the caves many times before should be declining their help.
"Civil protection rescuers are at the scene, but the six Englishmen have refused all help as I imagine they don't trust in the abilities of the Mexican rescuers," said Jose Trinidad Luna Reyes, the Puebla Red Cross spokesman.
But the MoD gave every indication last night of quietly relishing the idea that the expedition had, by chance, turned out not to be plain sailing. "This is adventurous training, to test leadership, stamina and self-reliance," said a spokesman last night.
"There has to be some element of risk. You don't put off climbing the Matterhorn because it'll be cold and snowy; you have to go and do it anyway."Reuse content