Kennecott, just outside Salt Lake City, has been operating since 1903 and the Bingham Canyon pit is the world's largest man-made, open pit excavations in the world, visible from space.

Initial underground mining will focus on an area known as the Lower Commercial Skarn (LCS), which will deliver a total of around 30,000t of copper from early 2023 through to 2027.

LCS has a resource of 7.5 million tonnes at 1.9% copper, 0.84 grams per tonne gold, 11.26gpt silver and 0.015% molybdenum and a reserve of 1.7Mt at 1.9% copper, 0.71gpt gold, 10.07gpt silver and 0.044% molybdenum.

Full underground production is expected in the second half of next year.

"This investment will allow us to quickly bring additional volumes of high-quality copper to the market and build our knowledge and capabilities as we evaluate larger scale underground mining at Kennecott," Rio Copper chief executive Bold Baatar said.

"We are progressing a range of options for a significant resource that is yet to be developed at Kennecott, which could extend our supply of copper and other critical materials needed for electric vehicles and renewable power technologies."

The company is also trialling underground battery electric vehicles, supplied by Sandvik Mining and Rock Solutions.

"Trialling underground battery electric vehicles is an exciting step in our work to create a safer workplace for our employees, increase the productivity of the mine and reduce emissions from our operations. We look forward to seeing their potential for deployment," Baatar said.

Existing underground infrastructure is currently being extended to enable early access to the next underground resource and undertake characterisation studies.

A feasibility study to inform decisions on the next phase of underground production is expected to be completed in 2023. This will be one of several potential stages currently being investigated.

Feasibility studies are also being progressed to extend open pit mining at Kennecott beyond 2032.