What is a mass extinction? how many have occurred in the last 600 million years?

Starting about 360 million years ago, a drawn-out event eliminated about 70 percent of all marine species from Earth over a span of perhaps 20 million years. Pulses, each lasting 100,000 to 300,000 years, are noted within the larger late Devonian extinction

the man is bald so he has no hair to get wet