Old dams trap a mountain's history in mud
By drilling into the massive silt deposits behind aging Japanese dams, scientists can read a year-by-year record of every typhoon and landslide the mountain has suffered.
When engineers built massive concrete dams across Japan's steep river valleys in the mid-20th century, they intended to harness water power and prevent flooding. Instead, they inadvertently created the world's most precise geological clocks. Because Japan is one of the most tectonically active places on Earth, its mountains are constantly shedding skin in the form of gravel, sand, and silt. These sediments, which would normally wash out to the Pacific Ocean, hit the still waters of a reservoir and sink, stacking up in neat, chronological layers that can reach tens of meters thick.