You stop

be sure to dig a path out, else with a broken pickaxe you will be stuck and forced to respawn and make tools to recover your gear

Originally posted by _I_:

be sure to dig a path out, else with a broken pickaxe you will be stuck and forced to respawn and make tools to recover your gear

Or just run up the wall while jumping. If there's any slope at all, you can get out.

You have to start swimming

1. The bedrock largely mirrors the surface 16 units down.

You can only dig down 8 meters from the point where you started digging, which is 16 swings of the pick.

The same is true building up the ground (such as building earthen wall) you may only build up 8 meters or 16 swings, from the point of natural ground where you started building up.

Thanks