Coal-Mine was a browser-based economic game. bitcoin mining game site. that in the [ full Bitcoin Games Site Watchlist ]. very simple animate mine field. and simple rule. you can collect free reward coal (250 satoshi) every hour.
but if you want mining fast. you have to buy the mine tunnel. since the most cheaper mine tunnel need 0.01 BTC. and the min withdraw amount is 25,000 satoshi + 3,000 satoshi (fee).
so if you want depend the reward buying tunnel. it will take some time.