What happens to jettisoned fuel tanks? When an F-15 (or your favorite fighter aircraft) punches it's external tanks...


...to engage in combat, (or when they are empty), what happens to them?

Through my best reasoning skills and Newton's calculations, I have determined that they will fall.

But what happens when they hit the ground? Do they explode? Do they detonate at a certain height? Is this a problem? Are there regulations determining where/when pilots may pickle their tanks? Have there ever been any accidents? (E.g. houses/cows/grandmothers destroyed by falling fuel tanks?)
They are called drop tanks. When a drop tank is jettisoned, very likely it is not retrieved and reused. This is also the case with (sadly, discontinued) Space Shuttle external tank. Those tanks were not reused either. I am not talking about the rocket boosters.

What happens to jettisoned fuel tanks?

It depends on where they land. The tanks jettisoned in Vietnam have been used to make boats (more pictures here).

It is worth noting that these external fuel tanks have disadvantage too:


  A general rule is that only about half the capacity of a streamlined
  drop tank actually goes towards increasing the aircraft's overall
  range, the rest going to overcome the added drag and weight of the
  tank itself.