  Active matter is rapidly becoming a key paradigm of out-of-equilibrium soft
matter exhibiting complex collective phenomena, yet the thermodynamics of such
systems remain poorly understood. In this letter we study the nonequilbrium
thermodynamics of large scale active systems capable of mobility-induced phase
separation and polar alignment, using a fully under-damped model which exhibits
hidden entropy productions not previously reported in the literature. We
quantify steady state entropy production at each point in the phase diagram,
revealing characteristic dissipation rates associated with the distinct phases
and configurational structure. This reveals sharp discontinuities in the
entropy production at phase transitions and facilitates identification of the
thermodynamics of micro-features, such as defects in the emergent structure.
The interpretation of the time reversal symmetry in the dynamics of the
particles is found to be crucial.
