Fifteen years of observations by the Gamma radiation telescope Fermi have revealed a crack in the floor of the Galaxy. South of the cluster Westerlund 1 — the most massive young star formation cluster in the Milky Way — extended emission has been detected, coinciding with a cavity in atomic hydrogen. This is not just another bubble blown by winds and supernova. It is an emerging outflow: relativistic electrons, like water finding a gap in concrete, burst through the dense layer of interstellar medium and escape into the halo. The crack is widening. The gas gives way.

The shock wave where the stellar wind collides with the surrounding gas — the plasma termination shock — acts as a natural accelerator. Here, particles are accelerated by the mechanism proposed by Enrico Fermi: multiple reflections off magnetic inhomogeneities at the shock front pump energy into them. This is how cosmic rays are born. The faint gamma-ray emission detected by the telescope is not direct light from the particles, but the echo of their interaction with photons: inverse Compton scattering, discovered by Arthur Compton. A fast electron strikes a quantum of the cosmic microwave background or interstellar light and turns it into a gamma-ray photon. The spectrum cuts off. Sharply. This gives a transport time: 125–200 thousand years from the cluster to the edge of the visible crack. The energy density of electrons in the flow is 1–10 eV/cm³, an order of magnitude above the background.

Massive stars live bright and die young. Their stellar evolution ends in explosions that, together with stellar winds, churn up the surroundings. Such cracks are windows into galactic evolution. Cosmic rays, accumulating in the halo, press on the gas, regulate star formation, and can even trigger large-scale winds. Westerlund 1 demonstrates this process in action for the first time, and at an early stage: the flow has not yet completely broken through the disk, but it has already created a visible breach. The particle pressure inside it is sufficient to influence the dynamics of the outflow itself. One gap — and the whole picture changes.

The crack in the Galaxy's floor does not heal — it becomes part of a giant ventilation system. Future observations, especially in the radio range, will allow measuring the synchrotron emission of electrons and directly estimate the magnetic field in the outflow. Next-generation telescopes, such as CTA, will see the high-energy tail of this process in detail. And three-dimensional modeling will explain how turbulence and magnetic fields shape the escape channels. This is the moment of birth of a galactic wind — the floor beneath the Galaxy cracks.