* they're derived from an estimated probability of the event (independently) happening each year. It doesn't mean that it won't happen for n years. The probability is the same every year.

* the probabilities are estimates, trying to predict extreme outliers. Usually from less than 100s of years of data, using sparse records that may have never recorded a single outlier.

* years = 1/annual_probability ends up giving large time spans for small probabilities. It means that uncertainty between 0.00001% and 0.00002% looks "off by 500 years".

https://practical.engineering/blog/2025/9/16/an-engineers-pe...