Is it a giant cocoon? The site of a pagan ritual? Or a 60-tonne pebble?

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A bulbous white cocoon, still sticky with the excretions of whatever creature made it, has landed in Kensington Gardens. Perched atop a ring of boulders that lie strewn around a hollow in the landscape, it looks like the site of a momentous pagan ritual. Could there have been a solstice birthing of some strange animal from this great chrysalis, which has been torn open in places where it slithered out?

Whatever the story behind it, this year's Serpentine pavilion is the weirdest structure yet to have graced the west London lawn. It is the work of 48-year-old Chilean architect Smiljan Radic, the least-known of the menagerie of global designers who have been invited over the past 14 years of this experimental annual commission – and he has crafted something unique.
Chilean architect Smiljan Radi designed the fourteenth Serpentine Pavilion which opened this morning
60 tonnes of rock scattered like gravel ... Radic's pavilion. Photograph: Sarah Lee for the Guardian Copyright Sarah Lee www.sarahmle/Guardian

"I wanted to make it look like it came from the hands of a giant," says Radic, standing in front of 60 tonnes of rocks that seem effortlessly scattered like gravel. "In the tradition of the English garden folly, it should be something that surprises the public and draws their attention, providing a spatial experience that you don't get every day."

Unless you are familiar with life as a moth in the pupal stages, it is unlikely you'll have been in anything quite like it. Layering fibreglass sheets over a doughnut-shaped mould, Radic has built up a translucent shell of impossible fragility, forming a skin just 10mm thick, which seems to hover above its rocky foundations.

"In England, you build with lots of layers of structure and insulation and waterproofing," he says. "I'm interested in more fragile structures, creating an enclosure with a single thin skin." During the day, this rough-cast blob, which was made in Yorkshire, has the solidity of a pebble. But once inside, or when seen by night, it glows with a yellowish tinge, its fibrous surface giving it the look of shed skin. It is jagged and smeary, a texture offset by fine steel wires that hang a zigzagging lighting rail through the space, and an angular window, sharply sliced out to capture views of the lake beyond. It is a careful assembly of things that are both ragged and refined, contrasting roughly hewn with smoothly polished.
Serpentine pavilion
A shell of impossible fragility ... Radic's pavilion. Photograph: Sarah Lee Copyright Sarah Lee www.sarahmle/PR

The project started life as a papier-mache model, strips of paper layered over a rubber ring, then torn open to create views through the looping cave. "I don't have fine movement in my hands," says Radic, "so I like to cut and destroy things more than compose something. I'm not a creator of forms, so my method is to collect different possible shapes and use things as I find them."

His studio in Santiago is full of such found objects, from stuffed cow's udders wrapped in masking tape to hollowed out rocks and pebbles with natural holes in them – one of which provided the inspiration for the pavilion. Inspiration also comes, he says, from David Hockney's etchings of Grimm fairy tales and the writings of Oscar Wilde and Proust, as well as the methods of his sculptor wife, Marcela Correa.
Serpentine pavilion
'A Chilean miracle...' the architect inside his cocoon-style pavilion. Photograph: Sarah Lee Copyright Sarah Lee www.sarahmle/PR

Like the pavilion, his work in Chile revels in material juxtapositions, contrasting geological power with the delicacy of things that feel woven or grown. He has built a restaurant in Santiago of fine concrete beams that rest on massive granite boulders – a touch of Mies meets Flintstones – and a concrete bunker of a house, lined with a warm wooden interior. Elsewhere, his baked-in-situ house for a charcoal-burner could have been rolled into shape by a dung beetle. In each case, his approach to architecture prioritises materials and atmosphere over form and facade.

"He's part of the Chilean miracle," says Serpentine curator Julia Peyton-Jones, who first spotted Radic at the Venice Biennale in 2010, where he exhibited a 14-tonne boulder, scooped out with a little wooden nest inside. "Santiago has 40 architecture schools, and building is much easier there, which breeds a real fluency of making."

"It's the mountains and the sea," adds co-curator Hans-Ulrich Obrist. "Like Japan and Switzerland, these constraints produce great architecture."