A group of motivated cavers are hoping to extend the country's longest cave, to make it the longest in the Southern Hempishere.
Thirty cavers have embarked on a 17-day expedition in the Bulmer Cavern, which runs for about 70km through Mount Owen, in the Tasman District.
The team was also planning to film a documentary for the Wanaka Film Festival.
Cave diver Kieran Mackay said the group had the potential to make the cave much longer.
"Part of that is actually diving, some teams are going to be climbing, some people are going to be digging up passages that are full of sediment and everyone just wants to make the cave longer," he said.
"And there's potential for a really massive, massive cave system that's much much longer than the 70km that we've got already.
"When you see the map, and you see the size of the Bulmer system and the size of the rest of the mountain, you've got this 70km piece that's not very much of the mountain, then there's a massive mountain to go through.
"It's got potential to be 100-plus, maybe 150 some people are saying even longer than that, I don't know.
"You never know with caves, it'd be very nice if it became the longest in the Southern Hemisphere."
Mr Mackay said they would need a range of kit including food, diving and camping gear.
He said they would be travelling 8km through the cave before diving through an underwater passage.
"We've got no idea how long it will be, it may by 10 metres, might be 20 metres, might be 100 metres long, I don't know. We're just hoping it won't be deep, as we're not prepared for a deep dive."
"So we're looking forward to getting through that dive and hopefully through many many kilometres of new passage."
The current longest cave in the Southern Hemisphere is the Clearwater or Gua Air Jernih cave in Malaysia, which runs for about 215km.
He said the first stint of being underground without daylight would last for eight days.
He said the storm coming in over the next couple of days would mean needing to stay away from the main streamways. Water and loose rock could be big problems.
"It's a big psychological thing for people, because going underground for long lengths of time away from daylight is quite hard for people, especially when you've got a long way from daylight and you know you've got hours and hours to get back."