'That was a National Geographic jaunt to the B-15 iceberg, which at the time was larger than Jamaica.

'The aim was to become the first people to cave dive in an active iceberg. To say that it was fraught with danger is an understatement.'

The 2001 trip was denied endorsement by the United States National Science Foundation because they deemed it too dangerous.

As a result they had to travel under the flag of New Zealand whose government accepted the risk of the party.

Travelling the 12 days by boat from New Zealand to the Ross Sea area, Ms Heinerth and her team, including National Geographic photographer Wes Skiles, entered the dramatic caves.

'The thing with the iceberg was tha...

'That was a National Geographic jaunt to the B-15 iceberg, which at the time was larger than Jamaica.

'The aim was to become the first people to cave dive in an active iceberg. To say that it was fraught with danger is an understatement.'

The 2001 trip was denied endorsement by the United States National Science Foundation because they deemed it too dangerous.

As a result they had to travel under the flag of New Zealand whose government accepted the risk of the party.

Travelling the 12 days by boat from New Zealand to the Ross Sea area, Ms Heinerth and her team, including National Geographic photographer Wes Skiles, entered the dramatic caves.

'The thing with the iceberg was that it was constantly moving,' explains Ms Heinerth who works as a professional photographer and filmmaker in Florida.

'Entrances and creases were opening and closing as the iceberg went through the motions.

'What didn't help as well was the -1.2 degree sea temperature, that was something else.'

Usually carrying up to £30,000 of torches, cutting tools, rope, special 're-breather' aqualungs in triplicate on her cave journeys, Ms Heinerth's motto is to never be unprepared.

Due to the delicate nature of cave diving even a series of bubbles released from her aqua-lung could cause rocks or ice to dislodge and trap her.

'The most obvious thing that could go wrong is to get stuck, in the dark and without any idea which way you are facing,' says Ms Heinerth.

'It sounds glib, but do not panic. If you do you increase your chances of dying dramatically. It is a real test of mind over matter.'

Stunned by her experiences in the pristine, unexplored Antarctic, Ms Heinerth's fame in the cave diving world has grown since the mid 1990s.

'I have been scuba diving for twenty years, but when I broke the women's world record for distance travelled underground, underwater, then my career really took off,' says Ms Heinerth.