Repentagram x2

When the Black Mesa facility was
being developed, experiments were
undertaken with construction of a
five sided structure that was 
designed in such a way that the 
entire building would act as a giant
transmitter/receiver.  The building
was nestled in the rocks near the
Black Mesa Research Facility and
quickly used as a headquarters
for the invading military assault
teams.  Although the Black Mesa
blueprints don't show it, a second
pentagram shaped tower was built
near the first on the other side
of a thick rock wall.  The civilians
hoped they could hide in the secret
second tower until the chaos was over,
but soldiers quickly discovered the
structure and blew holes through the
rock wall to allow them to attack.
