A new study has revived a controversial hypothesis suggesting that Earth's greatest geological crises may follow a natural cycle that has repeated itself for hundreds of millions of years. Researchers reviewed previously published geological databases containing records of 89 major events that occurred over the past 260 million years. The research, published in the journal Evolving Earth, revisits a hypothesis that has sparked debate among geologists for decades. Now, geologist Michael Rampino of New York University has re-examined the evidence using updated geological timescales and more precise statistical methods. The ages of several of the largest known impact craters on Earth appear to be consistent with the proposed 27.5-million-year cycle.