[urq] ignition system electrical question

[Mike Fitton]

No Audi-specific content here, but something's been bothering me lately. 
  Okay, the coil generates a spark that's like 50 billion volts or 
something (or at least a lot more than 12) and sends that out, through 
the distributor, through the plug wires, through the plug.  I kind of 
understand where this voltage comes from...the sudden collapse of a 
magnetic field in the coil which creates something that probably has a 
much more technical term but is essentially an electrical whiplash...the 
same sort of thing that relays without diodes can do that destroys 
switches over time.  Now, what I don't get is where this voltage goes 
after the spark plug.  It seems like it would just ground to the engine 
and then eventually the battery.  But how can this kind of voltage 
ground to a 12v negative?  BTW, anyone know exactly how much voltage and 
current is actually taking place here?

-Cheers!
Mike