Sadly, when you all come back here in 50 years, we'll be doing this tour by helicopter
"What would happen if it collapsed now?"
We were placidly bobbing on New Zealand's Tasman Lake, staring up at a wall of ice.
"Well, nothing, to be honest. The lake is so deep, we wouldn't notice the wave out here. On the bank, it's a different story. If some ice fell in now, we're possibly in the safest place you could be - from the tsunami that is. Different story if the glacier calves from underneath."
This was our guide, Steve, a man of reassuring optimism. Nevertheless, the couple in the front of the craft huddled a little closer.
Steve had expertly steered our small rib through the rafts of ice to our current berth, a short distance offshore from the Tasman Glacier - the country's largest at 600m thick and 27km long. It was like something out of Game of Thrones, sheer cliffs of ice rose from the murky deep to tower above our heads.
And it was blue. I had seen ice before back home, in the frozen puddles and tractor ruts that riddle Wiltshire's countryside, but nothing came close to matching this shade of azure.
"It's to do with the lack of air bubbles." Steve explained, "The snowfall crushes the underlying ice, squeezing out the air. This purer ice only reflects blue wavelengths."
Steve turned our attention to a nearby iceberg that had broken off from the glacier.
Up close, the size of these floating peaks was truly intimidating. It was inconceivable to imagine the other 90% concealed underwater, and daunting to contemplate the danger this hidden portion posed. Submerged ice fans out from the iceberg in a shallow shelf below the waterline, and the shelf's greater buoyancy means it yearns to break free from the main core and rise to the surface. When it does, chaos ensues.
"If the surfacing slabs don't get us, the main iceberg will - pretty unstable after such a trauma, like as not she'll flip over. And this ice is a lot denser than the stuff in your fridge, we'd have no chance! But still, a collapse from the main glacier is a whole different situation."
Big collapses, or 'calvings', occur when the unstable end of the glacier breaks off into the lake, and with glaciers such as the Tasman these happen about once every Summer.
"Though this season we've had two already, which is strange." Steve broke off and frowned at the glacier.
So when was the last calving?
"Oh, two nights ago in fact!" Steve recalled.
A silence descended on our vessel.
"But don't worry," he added quickly, "calvings don't usually occur so soon after one another." tactfully defusing the tension in the boat, if not the glacier. For the Tasman is fast disappearing. In the 1990s the annual retreat was 180m. Today, the ice melts at a shocking 800m per year.
"It's a shame," mused Steve, gazing up at the root of the glacier high up in the mountains. He glanced at some kids in the back of the boat.
"Sadly, when you all come back here in 50 years, we'll be doing this tour by helicopter."
Ecologist 'New Voices' Travel blogger Robert Trevelyan has just graduated from Bristol University with an MSc in Geology with a special emphasis on environmental issues such a climate change and anthropogenic forcing. He is planning a career as an environmental journalist.