Geological time - that's what French artist Abraham Poincheval is hoping to experience over the next week, as he is entombed inside a limestone rock.
He will become the "beating heart" of the 12-tonne boulder, he says - discovering how that perspective warps time and space.
Poincheval is conducting his experiment at Paris's Palais de Tokyo gallery.
A space the shape of his body - and air holes - have been carved into the rock.
There is also a place for supplies of water, soup and dried meat. Cables will monitor his heart and provide an emergency video link.
Journalists looked on as workmen pressed together the two halves of the rock at the gallery on Wednesday.
"The purpose is to feel the aging stone inside the rock," Poincheval told Quartz Media during his weeks of preparation for the ordeal.
"There is my own breathing, and then the rock which lives, still humid because it was extracted not so long ago from the quarry. So there is that flow, that coming and going, between myself and the stone."
At the time, he was coy about how he would go to the toilet, but in the moments running up to his entombment he came clean to AFP news agency, admitting he would urinate into empty water bottles and sit on a small container to excrete.
And the smell?
"The stone will absorb some of the smell," he said. "I think I can take it."
Instead, he says, one of the biggest challenges will be psychological: keeping grasp of reality.
Poincheval is an old hand at putting his body through the bizarre and potentially perilous - often within a confined space, having previously spent 13 days inside a bear.
If he emerges intact from the boulder, Poincheval plans to sit on a dozen eggs for the three or four weeks they take to hatch.
And his ultimate ambition is to "walk on the clouds", he told AFP.
"I have been working on it for five years, but it is not quite there yet."
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