Hawaii’s Nīnole Hills, jutting out from Mauna Loa’s southeast flank, are one of the most striking features on the Big Island, though their geologic origins have long been a mystery. A new study looking at gravity anomalies under the hills is revealing that the hills are part of an older rift system that predates the currently active Southwest Rift Zone. The seemingly sudden switch from one rift system to another may provide some clues as to how Mauna Loa grew to be the largest volcano on Earth. “A lot of different ideas have been proposed to explain how the Nīnole Hills were created,” says Jeff Zurek, a geophysicist at Simon Fraser University in Burnaby, British Columbia, and lead author of the new study published in Geophysical Research Letters. It’s been suggested that the hills could be the remnants of an older summit of Mauna Loa, or its predecessor, Mohokea, or that faulting and landslides could have created the unusual topography of the hills, or that they could be from an older, inactive rift system.