The shielded room is made from two layers of wooden frame covered with sheets of low coercivity metal. Afrer each frame is placed the metal is magnetized. So, in fact, the shielded room is a giant magnet (9000kg) carrying a magnetization parallel to Earth's field. The effect of this structure is a uniformly low field inside the box. How low? before assembling the room the averaged field in the lab was 43,280 nT. After the shield was built the averaged field was reduced to 169nT (0.39% of the ambient field).
The room was built by Dr. Gary Scott and his team from Lodetar Magnetics Inc. on January-February 2016.
Detailed explanation on the physical principles of active shielding are in this article: Scott and Frohlich (1985).