  The paper studies a dynamic blocking problem, motivated by a model of optimal
fire confinement. While the fire can expand with unit speed in all directions,
barriers are constructed in real time. An optimal strategy is sought,
minimizing the total value of the burned region, plus a construction cost. It
is well known that optimal barriers exists. In general, they are a countable
union of compact, connected, rectifiable sets. The main result of the present
paper shows that optimal barriers are nowhere dense. The proof relies on new
estimates on the reachable sets and on optimal trajectories for the fire,
solving a minimum time problem in the presence of obstacles.
