The University of New Brunswick maintains an Earth impact database and here is a world map from their website which has plotted on it 172 verified impact craters on Earth. They are denoted by the little white dots. Let us look at where craters are and where they are not. You will see that northern Europe is well represented and North America has a lot of craters on it. Australia has quite a few. The other continents have a smattering. There are none in Antarctica or Greenland. The Amazon basin has hardly any. The ocean floor doesn’t really have any either. There are two reasons why this could be. One is that the Earth is preferentially hit in certain places by asteroids. Or, it is just an artifact of the way things get preserved or not preserved. We will see that actually it is the latter. It turns out that the ocean crust is too thin to preserve the evidence of an impact and also the oldest ocean crust is only about 150 million years old. That means any asteroid that hit the Earth in the oceans, which accounts for most of the surface of the Earth, any time before 150 million years ago couldn’t possibly be preserved because there is no ocean crust left over from that time period due to the recycling action of plate tectonics. In places where there is a huge sediment cover and things are hard to get to, like the Amazon basin, you just don’t have preservation because nobody has been able to find it. Same with Antarctica and Greenland. Under the ice we probably do not know what is there and what is not. This kind of gives you a clue about why it is hard to find ancient craters and correlate them with times of mass extinction events in the history of the planet. We have an imperfect preservation record of where a crater could possibly be.