TiCo hammer. All Stone, Diamond Tip, Three levels of Redstone. Now you can blow a 3x3 tunnel through the world rapidly, the longer you go, the faster you go. And with all the cobble you generate, you're never out of repair material. You can also sub in a single piece of lapis for the third level of redstone. It'll slowly accrue to Fortune 1, then 2, and 3.
Interestingly, if all you want are diamonds, you don't have to run turtles from the surface. Excavate works perfectly well from level 16.