There is a crack, like lightning. A shard of ice the size of six double-decker buses slowly begins to splinter off the face of the glacier directly in front of where we are standing. Gaining momentum, it smashes into the water with explosive force. Waves ripple across the lake. Icebergs bob. And the crowd goes wild.

Seventy-eight kilometres from El Calafate in Argentina, Perito Moreno glacier is one of Patagonia's biggest show-stoppers. Covering 250 kilometres, it is a massive and beautiful beast: five kilometres wide, it peaks 60 metres from the watermark, with a total ice depth of 170 metres. Part of the world's third-largest freshwater reserve, the glacier creates a natural dam, splitting Lake Argentine in two.

Most people view the glacier from a series of balconies built where the glacier pushes up against the land, but boat trips are a popular alternative. I've chosen something a little more challenging – and hopefully rewarding – a seven-hour hike to the centre of the glacier.

Run by a local company, Heilo y Aventura, the Big Ice tour offers visitors a chance to explore Perito Moreno glacier by foot. After viewing the glacier from the lookout, participants are transferred by boat to the far side of the lake. There we are fitted for crampons before hiking one kilometre inland through dense forest and then heading on to the ice.

Although the glacier seems rigid and still, it is always in a state of flux. Each day it shape-shifts, its meringue-like peaks curling to the will of the wind, the internal lakes continually filling and draining, its face shattering spectacularly as it grinds up against land.

The shifting terrain, obvious cold and the famously unpredictable Patagonian weather can be a potentially lethal mix, so strict rules are in place to ensure everyone's safety. Participation is limited to those who are aged 18 to 45 and do no have any injuries. As the ice can be paper thin in parts, you must walk exactly where the guide tells you and keep pace with the group – there's no dawdling for snapshots. The guides are friendly but firm with these rules: anyone who doesn't obey gets pulled into line fairly quickly.

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Stepping on to the glacier is absolutely enthralling. The ice crunches underfoot at first, before it solidifies like polished marble. The glacier is mottled brown and discoloured from the hikers' shoes but as we trek further in, this gives way to a palette of creamy whites and bluish hues, caused by the ice absorbing red and yellow rays of sunlight and reflecting the blue ones.

We scramble up and down peaks and troughs, jump over glacial streams and tramp on ridges of frozen ice. It is hard going and often I'm left breathless. But I'm also grinning. I just can't quite shake the feeling that I'm essentially walking on water.

We stop for lunch by a glacial pool but there's one problem – there's nowhere to sit comfortably without getting a very cold, wet and numb bum. A plastic shopping bag provides some protection but we can't sit for too long as we simply get too cold.

After lunch we head deeper into the labyrinth of ice. Gaping open crevices and sinkholes fold and envelope around each other, their trenches glowing a deep glacial blue.

Nearby, a waterfall runs down into a sinkhole, the water washing and churning around its edge like a water slide. One of the guides hacks off a huge chunk of ice and throws it into the subterranean tunnel. We wait a full 10 seconds before we hear the echo of a splash. It's a bit scary to think how deep it is and how far we would fall if we slipped.

We each take turns posing for pictures in a small ice cave. While we wait, one of the guides disappears into a crevice before climbing up a sheer ice cliff using pickaxes and crampons.

The sun begins to dip and we turn back for land. By the time we climb into the boat, there's a chill through my body that five layers of warm clothing can't combat.

Luckily, the strong whisky that is being passed around can do the job. And they only serve their whisky one way in Patagonia: poured straight over ice that has been freshly shucked off the glacier that day.

TRIP NOTES

Hielo y Aventura runs the Big Ice trip from September to April. Participation is limited to guests between the ages of 18-45 who are healthy. Cost is 650 pesos ($196) including transfers. National Park entry fee (20 pesos) is extra.

See hieloyaventura.com. Other sightseeing options, including a smaller trek, are available.