In the Cave course, you'll encounter several Diglett popping out of the ground. To reveal the entire Diglett family, you need to be quick and precise.
Throw an Apple at the first Diglett you see. This will cause it to pop out further. Then, as you progress, keep an eye out for more Diglett. Each time you see one, throw an Apple at it. If you hit enough of them in sequence, a Dugtrio will eventually appear, offering a great photo opportunity.