mine

os.loadAPI("tAPIs") --loads fuel function
local depth = 0 --sets variable to return back to surface
function junk() --defines function
    if turtle.inspect() == "minecraft:iron_ore" then --determines if item is valuable
        turtle.dig()
    end
end
function walls() --checks all four walls and determines if anything is worth mining
    for w = 1,4 do --runs four times
        turtle.select(1) --selects first slot
        turtle.inspect() --compares front object to first slot
        junk()
        turtle.turnLeft() --turns left for next wall
    end
end
tAPIs.fuel() --refuels
turtle.digDown() --digs down so that detect doesnt return true
for d = 1,2 do --digs down two blocks
    turtle.down()
    turtle.digDown()
    depth = depth-1 --records depth
end
turtle.detectDown() --detects if block is under turtle
while turtle.detectDown() ~= true do --if nothing under turtle then this runs
    turtle.down()
    depth = depth-1
    walls()
    turtle.digDown()
    turtle.detectDown()
end

 if turtle.detectDown() == true then --when turtle encounters bedrock or air, needs to be fixed for caves
    tAPIs.fuel()
    while depth < 0 do --loop to return to surface or starting position
        turtle.up()
        depth = depth+1 --records depth
    end
end

if depth == 0 then --when turtle reaches starting position place down a block and stop moving
    turtle.select(2)
    turtle.placeDown()
    error()
end