Six members of the British armed forces have been trapped for five days 120ft underground in caves in Mexico, the Ministry of Defence has confirmed.
The six men are trapped by floodwaters near Puebla city
They were among a group of 12 from the Combined Services Caving Association on a unofficial expedition at the Cuetzalan caves, north-east of Puebla.
The six men are uninjured and in radio contact with their colleagues who escaped before water cut the cave off.
Divers may be sent in if the waters do not subside soon, said the MoD.
Two cave divers - one of whom has extensive experience of the Cuetzalan cave system - flew out to Mexico from the UK on Tuesday morning.
If the flood waters don't subside they will take in scuba diving equipment for the men.
The team was taking part in a caving expedition, codenamed Cuetzalan Tiger, that had the approval of the military but was not an official training exercise.
The cavers - who are trapped in the Cueva Alpazat - had expected the trip to take 36 hours.
"This type of expedition is called adventurous training. We encourage all members of the military with their own interests to develop them," said an MoD spokesman.
He said such activities helped to develop skills required in the military, such as leadership, endurance and survival.
He said morale was high in the caves, although the men were "a bit bored".
The spokesman said unforecast rain had flooded a low-lying section of the cave.
When the team realised the danger they were in they moved to a pre-prepared underground camp that had not been flooded, he added.
The team has food, sleeping bags and radio contact.
"This is the kind of thing all cavers know can happen," said the MoD.
He said the servicemen were in touch with a surface team by a land telephone cable
which they would have laid out behind them as they made their way through the subterranean complex.
They also had a special "mole phone" which worked like a mobile phone but uses magnetic currents in the ground to transmit a signal.
He said help had not been sent earlier because the men were following the correct procedure, which was to wait for the water level to fall.
But further rainfall prevented their escape, he said.
Those trapped have not yet been named as their families were still being informed.
The Combined Services Caving Association was formed in 1991, when it brought together caving in the Air Force and the Navy with the Army.