In the USA, requirements have, in recent years, become stringent regarding the need for removal of old underground storage tanks. Even a few years ago, people would decommission tanks by draining the fuel (gas or oil) and filling the tank with a concrete slurry. Recently, a government official told me that, today, any tanks discovered must be removed. That does not apply to tanks that were grandfathered in and were legally decommissioned under the old guidelines and the official did cite a possible exception to the current rule. He said that, if removing the tank would damage the home or something of real significance, like a valuable tree or an attached historical structure, then an exception might be allowed. Special permits would be required to exercise this exception to the rule.
As a realtor, periodically, this concern with tanks comes up as many banks won't close on a loan until any known underground storage tank is removed. Why? The issue is not the tank but possible pollution from the contents. If pollution has leached out from the tank, cleanup costs can be very expensive. I know of one such cleanup in my area, where the costs ran in excess of $50,000. That was more than one fourth of what the low end home was worth.
In just about every community there are licensed fuel tank excavation and remediation companies that can come in and locate, and evaluate and remove, any underground tanks. If you are a realtor, listing an older home, you might want to take a look around the house and the lot. Look outside, near the foundation, for any old metal vent pipes or fill tubes. In a basement or crawl space, look for small diameter copper tubing (it may be disconnected and rolled up) that comes through the wall -- usually a foot or so under the level of the soil outside. If you see such materials, you can anticipate and circumvent problems by having a professional locate and deal with any old underground fuel tanks prior to putting the home on the market. If you are suspicious that any such tank might be present, have the situation checked out. The cost, to have someone qualified look for old underground fuel tanks, is minimal. Now, if a tank is found, it costs something to have it removed. But, at the least, you will have resolved an ugly problem that was bound to come up anyway and at a much more inconvenient time.