We're probably going to get tier 4 & 5 islands in alpha, but if I understand right, what beta should give us is tier 1-3 islands, with something like 100-300m deep being tier 2-4, and 300m and down being tier 3-5. Basically, beyond tier 3, we're going to need to dig to underground caves and check them for their deposits. There's a specific tool called a Pulverizer that's not available now, and which removes very large quantities of stone/dirt voxels (without dropping the resources) to quickly hunt for such caves.
From what I remember there's two wipes:
Alpha -> Closed beta: lose everything, except templates
Closed beta -> Open beta: lose claims, keep inventory and templates
(and of course, "open beta"->"release" = what? we're on release now? since when?)