The chasm cutting an Antarctic base adrift
Thirty-six years after he first went there as a young meteorologist, BBC Weather's Peter Gibbs returned to the current, sixth incarnation of the British Antarctic Survey's Halley Research Station. There, on "a day with no horizon", he explored the chasm threatening to cast it adrift.
The Brunt Ice Shelf feels like another planet even on the sunniest of days, but when the cloud closes in it turns downright eerie as sky, snow and ice blend into one diffuse white light.
Approaching the chasm, the only hint of this 100m-wide gash is a neon glint of blue from the depths of a crevasse in its far wall.
The ropes laid by guides Matt and Al seem to vanish into thin air, but as I hook on and nervously reverse up to the edge I can see the coils lying on polished ice 30m vertically below.
The initial commitment is tough, but once suspended I start to enjoy the drama of the place, sliding down past annual layers of ice which count off the years since I was last in Antarctica.
At the bottom, an intense silence and then the thrill of tasting salt water 40km from the coast - the Weddell Sea oozing up through the crack as it widens at a relentless 15cm a day.
It's a sobering reminder of the precarious nature of life on a floating ice shelf.
Too soon, it's time for the long climb out - and a reminder that I'm not as fit as I was 36 years ago!
UK viewers can also watch Peter's Horizon programme - Ice Station Antarctica - on BBC Two at 8pm on Wednesday 4 May, and afterwards on the BBC iPlayer.