Money Rock
From Wild World Wiki
At the start of every new day, one of the rocks in your town is randomly selected to be that day's money rock. When hit with a shovel, this rock will throw out a sack of 100 bells. This first hit will activate a counter, during which hitting the rock repeatedly will make it throw out larger and larger sacks of bells. When the timer runs out, the rock will no longer release bells on being struck, and you will have to wait for the next day for a new money rock to be created. The bags of bells come out in the following order: 100, 200, 400, 800, 1000, 2000, 4000. This means a grand total of 8500 bells are available from any money rock.
Money Rock Strategies
One popular strategy is to dig three holes in a row behind yourself before you strike the rock. This will prevent the recoil from the strike, and so allow you to hit faster and ensure you collect all the bells available. A variation on this to dig three holes in a v-pattern, one to the southeast and southwest of the rock, and one behind you. This prevents almost any recoil, but can lead to difficulties getting all the bells.