A neutron star is a superdense pie, and its equation of state is the dough recipe. The stiffer the dough, the less the star deforms when squeezed. Observations of two gravitational-wave bursts — GW170817 (merger of two neutron stars) and GW190814 (a neutron star swallowed by a black hole)—gave conflicting hints: one required a soft recipe, the other a stiff one. Like obsessed bakers, scientists sifted through thousands of compositions and found a family of recipes where the filling becomes two-layered, and sound inside races at nearly the speed of light. Only a few dozen out of thousands passed the strict test of observations.
Thus they managed to reconcile the data on the merger of two neutron stars and the absorption of one by a black hole, without going beyond Einstein's theory.
If someone shouted inside a neutron star, the sound would circle Earth 7 times per second: its speed is comparable to the speed of light.
In Larry Niven's story 'Neutron Star,' the hero survives near such an object. Reality turns out to be no less exciting.