Crazy as it might seem, there was a time when the U.S., the Soviet Union, and other countries tested nuclear weapons by exploding them in the atmosphere. From 1945 to 1963, when such tests finally were banned by an international treaty, more than 500 nuclear bombs were detonated, releasing radioactive fallout that spread far and wide across the planet, causing harm to the environment and human health.
For example, everyone born in the U.S. after 1951 has been exposed to nuclear fallout, and for some, it's resulted in an increased risk of thyroid cancer, according to the U.S. Centers for Disease Control and Prevention (CDC).
But for scientists that fallout also has provided an important measuring tool. The tests caused what's known as the 14C bomb pulse because of the spike in the atmospheric concentration of carbon-14, a carbon isotope that also occurs naturally. The excess carbon-14 was distributed throughout Earth's atmosphere, peaking in 1963 when the test ban went into effect.
That radioactivity, which has gradually been declining since the 1960s, has been absorbed by plants, animals and people, creating a sort of time stamp that's enabled researchers to measure when things occur — from the longevity of white sharks to the growth of human knee cartilage and even brain cells. It's also enabled forensic investigators to estimate the age and year of death for human remains with much greater precision than previously possible.