(Old question: How much of mathematics is or could be done by the 'geniuses'?)

A lot of important theorems or even theories end up being named after or otherwise attributed to one person or a small group of people. This is often fair, but taken as an overall trend it can give the (hopefully false!) impression that the most important mathematics is being done by a small minority of mathematicians. What I'm wondering is, how much does the opposite phenomenon occur, where it's very clear that a result is a large team effort and that no small group of authors deserves the lion's share of the credit? The most obvious example I can think of is the Classification of Finite Simple Groups, but I'm sure there are others from different areas of maths.