A three-mile wide comet, crashing at 100,000 mph into the Pacific Ocean near San Francisco, could cause millions of casualties. It would generate a thermal radiation wave, a massive earthquake and a lethal mega-tsunami that could desolate coastal communities around the Pacific basin and instigate torrential rains.

It may have happened before. According to Columbia University geophysicist and oceanographer Dr Dallas Abbott, a massive three-mile wide comet may have crashed into the Indian Ocean, 930 miles southeast of Madagascar, 4,800 years ago, creating an 18-mile wide crater, 13,000 feet underwater.

Dr Bruce Masse, an environmental archaeologist at the Los Alamos National Laboratory, suggests that 50% of Earth’s population may have perished in the resulting mega-tsunamis and global torrential rain. Dr Masse argues that this catastrophe may have inspired legends such as the Biblical 'great flood'.

The Indian Ocean comet impact is a controversial hypothesis that is being researched by a multidisciplinary group of scientists. They argue that it is one of several oceanic impacts that may have occurred within the last 12,000 years, the so-called Holocene Epoch. If the hypothesis were to be demonstrated, it could indicate that massive cosmic impacts occurred much more frequently than previously thought; every one or two thousand years, instead of the currently accepted half to a million-year cycle. This would also mean that the next massive impact could be around the corner.

Comet impacts are practically unpredictable. Dr Donald K Yeomans, an astrophysicist at NASA’s JPL, explains that comets are very special cosmic objects, very different from asteroids. In the unlikely event that we do find a large comet that is on an Earth-threatening trajectory, there's not a great deal we can do to avoid a catastrophic collision.

Using animations, we follow a comet from birth to death, trying to understand its composition and lifecycle. Astronomer Dr Claudia Alexander explains how each comet has its own characteristics and personality, making it impossible either to make generalisations about them or to design ways to prevent a lethal impact. In the words of respected astronomer and comet hunter Mr David H Levy, “Comets are like cats. They have tails and they do precisely what they want”.

Finally, we depict the dramatic effects of a lethal comet strike near the coast of San Francisco. Using Computer Generated Imagery, we show how the three-mile wide comet is transformed into a fireball appearing in the sky, bigger than the Sun, crashing into the Pacific Ocean 100 miles west of the Golden Gate Bridge, sending a thermal radiation wave bigger than Nagasaki’s atomic bomb, followed by an 8.2 earthquake and a mega-tsunami big enough to drown every hill and bridge in and around the City by the Bay. But this would be just the beginning of a mega disaster that would have globally lethal and long-lasting effects.