After a while you will fall to the cave system . But in there has been flooded by water, so you need located where water comming then blocks it.
The first things you will see after fall down to the cave system is lots of coal blocks. But the things we must focus is a lonely diamond block in the left.
This mine cave system quite of complex then we can’t accurately determine the location of each diamond blocks, but if you try to searching carefully you can find 15+ diamonds blocks including some diamonds without having to dig.