The interesting thing is the physical process creating the floor is a fairly novel one, coherent scattering of neutrinos off nuclei. This is where the neutrino scatters off all the nucleons together, with the contributions adding coherently, rather than incoherent scattering off individual nucleons.

This process was first observed only fairly recently, at the Spallation Neutron Source using neutrinos from stopped pions. IIRC, they realized a service corridor under the target room would be unintentionally suitable for the experiment.

https://coherent.ornl.gov/