Why are there giant concrete tunnels in the desert?

Two black wholes colliding. Talking about two entities that surely know how to pull the blanket on their side. Two 4  kilometers tunnels with in each one a laser beam that bounce 300 times before converging to a common place to compare there travelling speed is something unusual. How do you even test that it works properly.  We have not reached the gravitational wave concept yet and I am overwhelmed. I feel so small! Just love it! Thank you for that awesome share.