Let's explore Boyle's Law in terms of diving.  Let's say a diver is
diving into a large tank about two hundred feet deep.  At the surface,
the diver takes a deep breath.  The air in the diver's lungs is then at
one atmosphere of pressure and fills the diver's lungs to capacity.  Then
the diver jumps into the tank.


As the diver descends, the pressure on the diver's lungs increases by
another atmosphere every 33 feet (about ten meters).  As they descend,
the diver's lungs are compressed to half, then a third, then a quarter,
and finally to a fifth of their original volume  once the diver reaches
the bottom of the tank.

But, let's say that the diver then takes in a full breath of compressed
air at the bottom of the tank.  As the diver heads back to the surface,
the volume of the air would double, then triple, then quadruple, and
finally quintuple at the surface because the pressure would be dropping
by one atmosphere every 33 feet.

It is for this reason that divers cannot hold compressed air when
returning to the surface; their lungs would simply explode.  Our diver
has had to steadily exhale the air as it expanded.  To avoid problems
associated with depressurization, a rule in diving is to never pass your
bubbles while returning to the surface of the water.