  Active Galactic Nuclei (AGN) inject large amounts of energy into their host
galaxies and surrounding environment, shaping their properties and evolution.
In particular, AGN jets inflate cosmic-ray lobes, which can rise buoyantly as
light `bubbles' in the surrounding medium, displacing and heating the
encountered thermal gas and thus halting its spontaneous cooling. These bubbles
have been identified in a wide range of systems. However, due to the short
synchrotron lifetime of electrons, the most advanced phases of their evolution
have remained observationally unconstrained, preventing us to fully understand
their coupling with the external medium, and thus AGN feedback. Simple subsonic
hydrodynamic models predict that the pressure gradients, naturally present
around the buoyantly rising bubbles, transform them into toroidal structures,
resembling mushroom clouds in a stratified atmosphere. The way and timescales
on which these tori will eventually disrupt depend on various factors including
magnetic fields and plasma viscosity. Here we report LOFAR observations below
200 MHz, sensitive to the oldest radio-emitting particles, showing the late
evolution of multiple generations of cosmic-ray AGN bubbles in a galaxy group
with unprecedented level of detail. The bubbles' buoyancy power can efficiently
offset the radiative cooling of the intragroup medium. However, the bubbles
have still not thoroughly mixed with the thermal gas, after hundreds of million
years, likely under the action of magnetic fields.
