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The 17 elements are: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, yttrium.

Rare earth metals are a group of 17 elements - lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, yttrium - that appear in low concentrations in the ground.

Since 2015, researchers have taken more than 470 samples of elements, including europium, holmium and scandium, from 60 sites across the Little Missouri Badlands.

Others hope to pry out the neutrino's mass by studying nuclei of holmium-163, a synthetic radioisotope, created with particle accelerators, that undergoes a process nearly the opposite of beta decay called electron capture.

Natterer and his team used atoms of holmium, a rare-earth metal, sitting on a sheet of magnesium oxide, at a temperature below 5 kelvin.