ELK CREEK, Neb. (AP) — The Canadian mining firm exploring a deposit of a rare heat-resistant element in southeast Nebraska says the latest samples are some of the most promising yet.

NioCorp Developments said Monday that tests of some of its latest drilling samples showed relatively high concentrations of niobium.

The company already added a third drilling rig that will help expedite this summer’s exploration of the site near Elk Creek, Nebraska.

The U.S. currently imports nearly all the niobium that’s used in this country to harden steel and make it more heat-resistant for industrial uses.

NioCorp previously estimated that more than 100 million tons of niobium rests several hundred feet below the ground. The company is trying to determine if it will be profitable to build a mine.