mine

local realDepth = 0
local rightTurn = true --If turning right will face the turtle in the right direction
local oneHigh = false

local xDistance = 0
local yDistance = 0
local zDistance = 0


function initialColumn(depth) --Dig the initial column. Digs down either until it reaches depth, or it can't go down
    for i = 1,depth do
        turtle.digDown()
        if turtle.down() then
            realDepth = i
            zDistance = zDistance + 1
        else --If it cant go down, it adjusts what realDepth is (How deep the turtle can go)
            print("Hit the end! i:")
            print(i)
                    realDepth = i - 1
            break
        end
    end
    for i = 1,realDepth do --Return to surface
        turtle.up()
        zDistance = zDistance - 1
    end
end

function turnaround()
    turtle.turnLeft()
    turtle.turnLeft()
end

function empty() --Empties inventory starting at slot 16
    turnaround()
    for i = 16,1,-1 do
        turtle.select(i)
        turtle.drop()
    end
 turnaround()
end

function digSlab(twoHigh, size)
    for x = 1,size do --Dimensions of the slab
        for y = 1,size do

            if (y == size) and not (x == size) then --When we hit the end
                if rightTurn then
                    turtle.turnRight()
                else
                    turtle.turnLeft()
                end
                digBlock()
                turtle.forward()
                if twoHigh then --If we need to dig two high
                    digUp()
                end
                if rightTurn then
                    turtle.turnRight()
                else
                    turtle.turnLeft()
                end
                rightTurn = not rightTurn --Change which direction to turn next time. This way the turtle will snake around
            else
                digBlock()
                turtle.forward() --Goes forward
                if twoHigh then --If we have to dig two high, also dig upwards
                    digUp()
                end
            end
        end
        if x == size then
            print("Done slab! Now figure out returning to origin")
            --Returning to origin
            if not rightTurn then --Face the correct direction
                turtle.turnRight()
            else
                turtle.turnLeft()
            end

            --March on back
            for x = 1, size do
                turtle.forward()
            end

            if not rightTurn then --Face back towards the front
                turtle.turnRight()
            else
                turtle.turnLeft()
            end

        end
    end
end

function digBlock()
    local success,data = turtle.inspect()
    if (data.name == "minecraft:sand") or (data.name == "minecraft:gravel") then --If theres sand or gravel in front of the turtle
        while turtle.dig() do --Dig as long as you can
        end
    end
    turtle.dig() --Try to dig, even if nothing can fall in front of it
end

function digUp()
    turtle.digUp() --Dig at first
    local success,data = turtle.inspectUp()
    if (data.name == "minecraft:sand") or (data.name == "minecraft:gravel") then --If theres sand or gravel above the turtle now
        while turtle.digUp() do --Dig as long as you can
        end
    end
end

initialColumn(4)
empty()
turtle.down()
digSlab(not oneHigh, 6)