Idiom for depth of knowledge is harder to obtain as one gets deeper

In some subjects it is easy to learn a small amount about it, but hard, or time consuming, to learn everything about it. Chess for instance, one can learn how the pieces move around in a few minutes. Learning some basic tactics in an hour or two. An opening in a day. Pretty quickly they’ll get to a point where each new step takes weeks or months to really understand and apply.

A more generic way to look at it would be "the law of diminishing returns as applied to learning". This however is overly fatalistic for encouraging conversation. Is there an idiom that describes this sensation? Perhaps an apropos quote?