  The most elusive and extreme sub-class of active galactic nuclei (AGNs),
known as BL Lac objects, shows features that can only be explained as the
result of relativistic effects occurring in jets pointing at a small angle with
respect to the line of sight. A long standing issue is the identification of
the BL Lac parent population, having jets oriented at larger angles. According
to the "unification scenario" of AGNs, radio galaxies with low luminosity and
edge-darkened radio morphology are the most promising candidates to be the
parent population of BL Lacs. Here we compare the large-scale environment, an
orientation independent property, of well-defined samples of BL Lacs with
samples of radio-galaxies all lying in the local Universe. Our study reveals
that BL Lacs and radio galaxies live in significantly different environments,
challenging predictions of the unification scenario. We propose a solution to
this problem proving that large-scale environments of BL Lacs is statistically
consistent with that of compact radio-sources, known as FR0s, sharing similar
properties. This implies that highly relativistic jets are ubiquitous and are
the natural outcome of the accretion of gas into the deep gravitational
potential well produced by supermassive black holes.
