Sons Of The Forest

After you have the shovel, (which you can get from ) I'd recommend going up into the snowy mountains where there are fewer cannibals, situating yourself a bit to have sleep, food, and water figured out, then just digging away. This is especially easier if the season is currently winter, as I believe that also slows cannibal spawn rates. The fewer cannibals, the easier time it is to dig. From there just keep digging till the achievement pops.

Step 1: Get The Shovel From Shovel Cave

Step 2:
Go To The Top Of A Hill To Start The Initial Hole,
Aim At Your Feet And Start Digging!
- Digging Slowly Pushed Me Down The Hill So I Just Had To Hold Down The Dig Button.
- Virginia Stood Watch As I Dug, Shooting Any Cannibals That Tried To Get Close.

Once You Get To The Bottom Of The Hill, Go Back To The Top. Repeat.
- Takes About 16 Straight Minutes Of Digging And The Achievement Pops.

You'll Also Get This One Along The Way:

Good Luck!

Because of something silly I found out the following:

Hold left click and just alt-tab the game. I keeps on digging. While it did that I just simply started watching a movie on my other screen. Achievement eventually popped.