There are lots of problems with old underground fuel tanks leaking. That's to the point that it's quite common to do drilling in the middle of city streets etc to check for leaked petrol, diesel etc.
I know of one situation (at work) where we had to relocate an electrical cabinet across the road and take quite a few measures to keep petrol vapour out. Yes, there's a service station across the road and obviously their tanks were leaking (it's since closed). Another one was simply a case of doing some digging and finding "oil" - it turned out to be diesel and further investigation found several old tanks on the neighbouring property. Leaks are common...
Several issues there. First one is you don't want to be leaking petrol and contaminating the ground and/or groundwater. A nasty environmental mess that's for sure. In the USA some fuel additives have been phased out specifically for this reason - they mix with groundwater and are extra nasty when that happens. And it's a very common problem.
Second, you don't want to be losing your petrol. Not at $1.50 or so a litre anyway.
Third, any leak of petrol is obviously a hazard if it's anywhere near a potential source of ignition.
So if the tank is made of steel then you want to make sure there's some proper corrosion protection so that it doesn't rust away and leak. I'd assume this would be done by tank coatings (eg galvanizing) and/or cathodic protection - either mains / solar powered or via the use of a sacrificial anode. Those methods are all common for protecting steel in hot water tanks, oil rigs, bridges, ships, gas and water pipelines and so on.
The tanks I've seen installed recently (at service stations) looked to be made of something other than metal. I assume it was fibreglass but I'm not certain on that point.