Honest question: How do you ground something that is already buried in the ground? Is there some sort of anti-corrosion covering that could act as a dielectric?
No, you have to connect to an existing ground after checking its resistance or dig a new one.
When digging the holes for new telco FTTC boxes replacing passive-only boxes, we have to measure the quality of the existing ground, usually done with either 5-10m of 2cm wide, 3mm thick flat steel about 30cm belowground, or with a steel pole rammed ~3m into the earth directly next to the old box.
Usually the flat-steel is corroded and not usable any more (because the earth simply dries out), while the 3m poles are still good enough to use them.
In case the "old ground" is unusable, we tear it out (shit, that can involve LOTS of digging) and place a 1x0.5m steel plate at 50cm belowground, so that the bottom border is at 1.00m.
In the case of building a new grounder in an urban environment, I'd choose ramming a pole. It's easier, especially in areas with loads of wires and pipework - the downside is you have to rely on sometimes decades-old plans which can be off as much as two meters, and ramming a steel pole is enough to rupture a water or, worse, a gas pipe.
Dry soil can be enough, but more likely, yes, there is probably an anti-corrosion coating. Either way, a ground clamp, a piece of #8-#4 bare copper, and an 8' ground rod, will solve the problem.
Copper? In plain earth? That will only give you complete corrosion in, depending on conditions, not even a whole year.
Usual standard is zinc-coated steel, 10mm (or larger, depending on stuff like attached lightning protectors, the diameter of the supply line and a number of other factors).
The connector wire is copper, sized from #8 to #4. The ground rod is 5/8" galvanized steel. That's the standard ground in the USA since water pipes went to PVC. And the copper will last underground indefinitely.
As mschuster91 said, there is always a risk of hitting existing lines when driving a rod in the ground.
Different types of the stuff we stand on have different resistances or conductivities. You can imagine solid granite is not as good a conductor as a farmers field of moist rich soil.
If a particular section of earth has poor conductivity it and has a charge you can imagine the charge can't escape. Maybe it is charged from lightning, a downed power line, friction in a pipe, etc. this is called ground potential rise.
If an area has low resistivity it can't create a steep potential gradient and the charge is spread out over the entire area.
a ground grid of copper conductor is buried under substations and power stations so that the ground on the entire premise is at the same potential. When a power line touches the ground the current flows back to the source through the ground to complete the circuit. If the ground has high resistivity and current flows through it there is a voltage difference from one point to the next. If those two points happen to be your feet you are a better conductor than ground. By laying the copper ground grid no ground potential difference can exist as the copper connects the whole premise with low resistance conductor.
