The role of the surroundings, or environment, in quantum mechanics has long captivated physicists’ attention. Recently, quantum phase transitions (QPT)– a qualitative change in the ground state as a function of a parameter– have been shown to occur in systems coupled to a dissipative environment. Despite the ubiquity of QPTs in contemporary theoretical physics, obtaining clear experimental signatures has been challenging. I start by presenting a recent experiment in which it was possible to thoroughly characterize a QPT caused by coupling to an environment. The system is a single-molecule transistor built from a carbon nanotube quantum dot connected to strongly dissipative contacts. The electrical conductance of this system is highly singular as T tends to 0: the conductance is 0 except at one special point (on resonance and symmetric coupling) at which electrons are fully transmitted with unit probability. I then turn to the theoretical understanding of this QPT obtained by mapping the problem onto that of a resonant Majorana fermion level in an interacting electron liquid. The unitary transmission obtained in the experiment is seen as a competition between the two leads. The deviations from unitarity at nonzero temperature are connected to residual interactions among the Majoranas; in this way, the experiment observes a signature of Majorana critical behavior.