We only watch about 10% of the sky at any given time... meaning an asteroid could sneak up on us(espically from behind the glare of the sun) and we may have as little as a week before impact. There is also a volcano on the island of La Palma called Cumbre Vieja, and possibly the next time it eruptes half of the island will slide into the Atlantic sending out a massive Tsunami with wave heights as high as 500 meters when the reach the US east coast.