There are very rarely any fuel storage tanks underneath the airport apron or taxiways.
The reasons for this are manyfold. I'll let you think about that, if you imagine the problems associated with burying tanks big enough to hold millions of litres, as would be required.
Also - the amount of internal tank support needed to support the heavy aircraft above them, the need to replace the tanks regularly (around 25-30 yrs maximum, even less if corrosion is detected), and the high potential for water ingress via condensation and flooding (many service stations have serious fuel quality problems with their underground tanks, due to pinholes in tanks, condensation, and flooding - which brings in dirt as well as water).
Major airport fuel supplies are normally held in what is called a "fuel tank farm". This is a number of very large, above-ground tanks that are interconnected, and the fuel is delivered via sizeable diameter pipelines buried under the apron.
Pipelines and wiring is all that is buried under the apron or taxiways - the soil under the apron, taxiways, and runways is heavily compacted to a serious depth - and the depth of the concrete is also substantial - anywhere from a minimum of 300mm to up to 900mm, if the underlying soil profile is poor.
Below is some reading on airport fuel storage design, that you may be interested in. If you aren't interested in the planning and calculations, you can scroll down to "Heathrow" where the Heathrow fuel storage arrangements are outlined.
www.iata.org - Policy - Documents - guidance for (airport) fuel storage