Question

why in petrol bunk petrol is stored underground

Answer 1

Because if they didn't there would be no room for the building or parking on the lot. They are also much safer being kept underground where they are less vulnerable to weather damage, tampering, or theft.

You need a large supply of petrol to refuel vehicles having these above ground means they are vulnerable to damage and hence to a serious accident/incident, leaving just pumps above ground minimises this.