https://www.science.org/ After passing through a security gate, the van descends into a tunnel that burrows under the forests of Olkiluoto, an island off Finland’s west coast. The wheels crunch on crushed stone as a gray, wet October day gives way to darkness. “Welcome to Onkalo,” deadpans Antti Mustonen, a geologist here. Onkalo—“cavity” or “pit” in …