Mississippi Frozen River, United States

Freezing is almost always an exothermic process, meaning that as liquid changes into solid, heat and pressure is released. This is often seen as counter-intuitive, since the temperature of the material does not rise during freezing, except if the liquid were supercooled. But this can be understood, since heat must be continually removed from the freezing liquid or the freezing process will stop. The energy released upon freezing is a latent heat, and is known as the enthalpy of fusion and is exactly the same as the energy required to melt the same amount of the solid.
Low-temperature helium is the only known exception to the general rule. Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be added to these substances in order to freeze them.