  Determining the architecture of multi-planetary systems is one of the
cornerstones of understanding planet formation and evolution. Resonant systems
are especially important as the fragility of their orbital configuration
ensures that no significant scattering or collisional event has taken place
since the earliest formation phase when the parent protoplanetary disc was
still present. In this context, TOI-178 has been the subject of particular
attention since the first TESS observations hinted at a 2:3:3 resonant chain.
Here we report the results of observations from CHEOPS, ESPRESSO, NGTS, and
SPECULOOS with the aim of deciphering the peculiar orbital architecture of the
system. We show that TOI-178 harbours at least six planets in the super-Earth
to mini-Neptune regimes, with radii ranging from 1.152(-0.070/+0.073) to
2.87(-0.13/+0.14) Earth radii and periods of 1.91, 3.24, 6.56, 9.96, 15.23, and
20.71 days. All planets but the innermost one form a 2:4:6:9:12 chain of
Laplace resonances, and the planetary densities show important variations from
planet to planet, jumping from 1.02(+0.28/-0.23) to 0.177(+0.055/-0.061) times
the Earth's density between planets c and d. Using Bayesian interior structure
retrieval models, we show that the amount of gas in the planets does not vary
in a monotonous way, contrary to what one would expect from simple formation
and evolution models and unlike other known systems in a chain of Laplace
resonances. The brightness of TOI-178 allows for a precise characterisation of
its orbital architecture as well as of the physical nature of the six presently
known transiting planets it harbours. The peculiar orbital configuration and
the diversity in average density among the planets in the system will enable
the study of interior planetary structures and atmospheric evolution, providing
important clues on the formation of super-Earths and mini-Neptunes.
