I found a pig spawner in one of the abandoned mines.
I'm guessing that the best thing to do in terms of resource collection is to dig deep to like 8-14 level, and then to look for a cave there (since caves have more resources than random)? Keep in mind that diamonds will be in veins that are sometimes diagonal and so you have to clear around them to recover all of them most of them?
Do caves actually contain more resources than normal or does it just appear that way because you can look through more surface area at once? Regardless, it would seem that it's a much faster way of collecting resources.