the coffin is found around 5-10 meters deep roughly. This gives you a key for the warehouse where you can buy upgrades to expand your cell, buy upgrades to give u longer to digging time, a better toilet for dirt, glowsticks and much more. the only difference is that you need cash to buy items. Cash can be earned in 2 places, a slot machine which isn't really worth doing and a guy in a cell who wants items that you find when you dig up and he can only be found in a cell on your way to have lunch.