The gravest threat we know of ...

Years ago, the Economist pointed out that based on risk assessment asteroid impact warranted as much attention as airplane crashes. That was then ...

Did an Asteroid Impact Cause an Ancient Tsunami? - New York Times

... Most astronomers doubt that any large comets or asteroids have crashed into the Earth in the last 10,000 years. But the self-described “band of misfits” that make up the two-year-old Holocene Impact Working Group say that astronomers simply have not known how or where to look for evidence of such impacts along the world’s shorelines and in the deep ocean.

Scientists in the working group say the evidence for such impacts during the last 10,000 years, known as the Holocene epoch, is strong enough to overturn current estimates of how often the Earth suffers a violent impact on the order of a 10-megaton explosion. Instead of once in 500,000 to one million years, as astronomers now calculate, catastrophic impacts could happen every few thousand years.

If the hypothesis is accepted, then the risk to life of a meteor impact will have increased about 500 fold. That would move it very high up the risk index, perhaps above thermonuclear war.

If that happens, and if humans were logical creatures, we would put a significant share of the world's GDP to surveilance and diversion research. Alas, humans are far from logical ...

PS. The 2807 BC rock landed on, of course, Atlantis. :-)