i just realized how you could basically mine infinitely down to bedrock using turtles, but without any fuel.
just have two turtles and a startup program for both of them. place one on top of one another. in both turtles, the startup program would dig the turtle above it, dig the block below, then replace it below it, and turn it on.
when the turtle gets turned on, its startup program runs, then that turtle digs the one that it just got placed by, digs below it, turns it on again and the cycle repeats.