Réunion, a French island in the western Indian Ocean, is a jigsaw of two massive shield volcanoes. The younger, Piton de la Fournaise or “peak of the furnace,” is a furious factory of lava, erupting every eight months on average over the last four decades.

That hellish environment makes it an ideal real-world laboratory for studying the internal viscera of volcanoes, about which scientists know surprisingly little. The more they map out, the better they grow to understand why, how and when volcanoes all over the world will next erupt.

In a study published this month in Scientific Reports, volcanologists reported using a novel technique to map out 58 square miles of Piton de la Fournaise’s shadowy underworld. Their survey revealed a 3D view of its insides, from the plumbing network of superheated hydrothermal fluids to scores of faults that allow magma to sneak up to the surface during eruptions.

The success of this technique on Réunion means that it could be deployed elsewhere, said Marc Dumont, a geophysicist at the Sorbonne University in Paris and the lead author of the study, from lava effusing mountains like Hawaii’s Kilauea to the more explosive peaks in the volcanic spine running up America’s Pacific Northwest.