The 47-year-old, known as 'the spider' for his climbing skills, has pleaded guilty to the crimes, which took place over 20 years.
"I thought it was very exciting. It was like a treasure hunt or exploration, so I continued to climb deeper and deeper into the mine, and found more minerals," he said.
The man, a former employee of the Norwegian Mining Museum, which controls the 30 heritage mines around Kongsberg, west of Oslo, was arrested as he came out of a mine shaft with a haul of silver ore in May 2012.
The man had secretly worked the mines for twenty years, beginning the year before they officially closed in 1992.
At his home, police seized silver with an estimated value of nearly two million kroner ($334,000), along with explosives and mining equipment.
"Of course it was a great satisfaction in that the first man we were able to expose was one of the big fish," Alfhild Skaardal, director of the Norwegian Mining Museum told TV 2.
Skaardal began investigating charges of theft when she came in as the museum's chief in 2009.