Ask them here. It's the underground support that a building's columns and floor slabs rest on.
Caissons go into holes in the soil that are drilled all the way to bedrock.
The exit near the P3 ramp facing Bldg 4 will have increased pedestrian traffic for quite a while. The crosswalk request was approved and now in place. National Institutes of Health, Warren Grant Magnuson Clinical Center, Bethesda, Maryland 20892.