  The Extreme Energy Events (EEE) experiment is the largest system in the world
completely implemented with Multigap Resistive Plate Chambers (MRPCs).
Presently, it consists of a network of 59 muon telescopes, each made of 3
MRPCs, devoted to the study of secondary cosmic rays. Its stations, sometimes
hundreds of kilometers apart, are synchronized at a few nanoseconds level via a
clock signal delivered by the Global Positioning System. The data collected
during centrally coordinated runs are sent to INFN CNAF, the largest center for
scientific computing in Italy, where they are reconstructed and made available
for analysis. Thanks to the on-line monitoring and data transmission, EEE
operates as a single coordinated system spread over an area of about $3 \times
10^5$ km$^2$.
  In 2017, the EEE collaboration started an important upgrade program, aiming
to extend the network with 20 additional stations, with the option to have more
in the future. This implies the construction, testing and commissioning of 60
chambers, for a total detector surface of around 80 m$^2$. In this paper,
aspects related to this challenging endeavor are covered, starting from the
technological solutions chosen to build these state-of-the-art detectors, to
the quality controls and the performance tests carried on.
