Scientists think that massive volcanic eruptions killed off more than 90% of ocean species and 75% of land species almost 252 million years ago. But they didn’t know when these eruptions occurred. Now, researchers have solved the mystery—using radioactive dating techniques to determine the ages of hundreds of uranium-bearing crystals taken from ancient volcanic rocks collected from sites scattered across a 2.5-million-square-kilometer region of central Russia. Some crystals came from material that had explosively erupted from Earth’s surface and then accumulated in layers hundreds of meters thick (such as those near the Angara River, shown), and others came from similarly voluminous magma that had remained underground but had infiltrated and heated carbon-rich material near the surface, creating carbon dioxide. The earliest phases of the volcanism began about 300,000 years before the onset of the end-of-Permian extinctions, the team reports online today in Science Advances. All told, an estimated 4 million cubic kilometers of molten material emanated from Siberian peaks and fissures over the course of about 800,000 years, with about two-thirds of that spilling forth before and during the mass extinction. Paleontologists estimate that Earth’s ecosystems didn’t robustly recover from the die-offs for as many as 5 million to 10 million years.