By Rebecca Morelle
Science reporter, BBC News
The subterranean secrets of badgers have been revealed by a BBC film crew.
Over two years, tiny cameras placed deep underground recorded the comings and goings of a wild badger family.
While the animals have been well studied outside of their setts, until now, little has been known about their behaviour while underground.
Never-before-seen behaviour was filmed, including the badgers diligently making their beds each evening before leaving the setts for a night of foraging.
The crew also captured newborn cubs on camera, as well as grooming and fighting between the older animals.
The footage was recorded for a BBC Natural World wildlife programme.
About 300,000 badgers live across Britain, their tracks and trails criss-cross the countryside, yet they are rarely seen.
Andrew Cooper, producer of Badgers - Secrets of the Sett, said: "Before we began filming, I knew that there was a gap in our knowledge about badgers.
"When I spoke to one badger expert, I said to him: 'How much do we know about their life underground?', and he simply held up a blank piece of paper.
"This is the first time we have had the opportunity to see real detail down there."
It took the team six months to prepare for filming, which took place in the Devon countryside. High-resolution, waterproof cameras, measuring less than 8cm (3in) in length, microphones and more than a kilometre of cabling were fed into the badgers' tunnels and chambers, which can be four or five metres deep.
Mr Cooper said: "The most difficult thing was putting the cameras in underground and then trying to get them into a position so we could see what they had got going on down there."
Even once the cameras had been manoeuvred into place, the team still faced several setbacks.
"Mice and squirrels conspired to nibble through various bits of exposed cables, and plants grew in front of the cameras," said Mr Cooper.
"The badgers would also come and lick the cameras or sniff them - one cub even tried to chew a camera."
The producer added: "Badger slobber all over the lens didn't do much for the vision."
Fear of thunder
During the many months of filming, the team managed to record these highly social, nocturnal creatures as never seen before.
Mr Cooper said: "One day, there was a huge thunderstorm. There were five badgers in this one chamber, all piled on top of each other. And right in the foreground of the camera, a cub of about six or seven months old sat.
"There was a huge clap of thunder - you could hear it rolling down the tunnels and around the chambers - and this little cub pulled its paws up over its ears. The images were riveting."
The team also discovered that the creatures were very clean and tidy while underground.
Before leaving their sett for an evening of foraging, they would "make their beds" by raking up their bedding and leaving it in a fluffy pile in the middle of the chamber.
The crew also managed to film the creatures fighting.
Mr Cooper explained: "During the summer months, the young males got fractious.
"Daylight lasts a long time, the nights are very short, and consequently they are itching to go outside."
Consequently, he said, the badgers would start moving around in the chambers and fights would break out.
All of this action took place in complete darkness, infra-red lights meant it could be captured on camera.
The producer added: "Their idea of spatial awareness just through touch, sound and smell must be quite extraordinary."
The Devon badger family still remain under surveillance - and can be seen on the Wildlink website's badger-cam.
Badgers - Secrets of the Sett is on BBC Two on Friday, 15 February, at 2000 GMT and is repeated on BBC Two on Saturday, 16 February at 1740 GMT.