Hey chief, if your boys are already past 90 feet below the surface, they would better be careful not to stay in that hole for more than five minutes continuously in any digging session. Otherwise they may suffer some breathing problems.
There is nothing you can do about the gas unless of course you install an air pump to pump out the gas. But again if you do that, you might suck almost all the air in the hole and your boys will have nothing to breath in.
...besides, the presence of a safe alone does not signify that there is money inside...