How much does it weigh? Does it leak? Does anything grow here?

For 35 years, Bill Dilworth tended a Manhattan loft filled with dirt, otherwise known as “The New York Earth Room,” a monumental artwork by Walter De Maria, a lion of Minimalism who died in 2013. And for decades, Mr. Dilworth, an affable abstract artist, patiently fielded those and other questions, noting the more intriguing ones, and the visitors who posed them, in a notebook he kept for that purpose.

“What is this for?” was a recurring question.

His answer: “It’s forever.”

The Earth Room is an extravagant, startling artwork — 280,000 pounds of dark, chocolaty soil, about two feet deep — on the second floor of an early artists’ co-op in a former manufacturing building on Wooster Street, in the heart of SoHo. It was installed in 1977, in what used to be the Heiner Friedrich Gallery, and it was intended to be temporary, a three-month-long exhibit. But Mr. Friedrich — who had formed the Dia Art Foundation (now the Dia Center for the Arts) in 1974, with his wife, Philippa de Menil, and others, as an organization dedicated to supporting work like Mr. De Maria’s — decided that “The New York Earth Room” should be one of its showpieces. It opened to the public, free of charge, in 1980.

Since then, the artists who colonized the building and the area have mostly moved on, and the neighborhood, like the city itself, has evolved.

“That’s what makes the Earth Room so radical,” Mr. Dilworth said in a video posted on the Dia website. “It’s here, and it remains the same.”

Mr. Dilworth was a bit radical himself: an artist who shunned the art world, but who spent most of his waking hours in one of its most important works.