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.

https://documents.geotracker.waterboards.ca.gov/esi/uploads/...