Huh, that's really surprising. How does it happen? I assume it has something to do with gasoline, but I was under the assumption that most of the gasoline at gas stations was stored in giant underground concrete containers.
Fuel tanks leak. Cars leak. Waste fluids (fuel, oil, brake fluid, transmission fluid, coolant, refrigerant) leak and are improperly disposed of. Fuel spills when being pumped in or out of tanks.
Until 1996, leaded gasoline was generally legal in the United States (California phased it out in 1992). It remains legal as avgas and possibly for some specialised uses. Tetraeythyl lead is a toxic heavy metal compound thought to be a factor in the rise in urban crime, a phenomenon which traces its use and phase-out closely, though lagged about 20 years.
Among the compounds used as an alternative to lead was MTBE (Methyl tert-butyl ether), an oxygenate and ground-water contaminant, itself banned as a fuel additive in 2004 (2002 in California), though again it remains legal for other uses.
Petrol (gasoline) itself is often formulated with numerous other additives, and "is a mixture of a large number of different hydrocarbons", averaging hydrocarbon length of about 5-6. ("Naptha", shorter chains make more volatile fuels, longer chains heavier, e.g., kerosene, diesel fuel, fuel oil, bunker oil. "Octane" is an 8-chain compound with lower volatility, increasing the ignition point and reducing engine knocking due to premature ignition.) Various of the naturally-occuring, added, and refining-induced compounds themselves may be harmful or toxic, and include VOCs.
Generally the tanks are fiberglass. I don't know why they leak. From reading site reports, my impression is these leaks are usually detected by the tax authorities, who can see that the reported sales of fuel don't add up to the reported deliveries of fuel. See page 40 of this report for an idea of what an underground fuel tank looks like.
There's an old illustration from Popular Mechanics that suggests dumping used motor oil into holes in the ground. At one time that was considered a reasonable way to dispose of it.