Here is the full list of all 14 Hidden Deep achievements.
Complete a level with high enemy presence without dying (must be 50+ enemies)
Drill 100 meters of tunnel using the Tunnel Boring Machines
Kill 1000 monsters
Ride a elevator from top to bottom non-stop. Yes, that's possible.
Kill 3 enemies using one Scan-Ball
Kill 5 enemies in a row while hanging on the grappling hook
Kill 20 enemies with your gun without dying
Let another team member use your rope to climb
Rappel down the full length of your grappling hook's line
Kill 100 monsters
Use the grappling hook as a zip-line
Find a hidden cave using the Terra-Scanner
Kill the ceiling tentacle before it kills you on your first encounter
Fall from 50 meters or more