A good safe must protect its contents not only against safecrackers but also against water, fire and natural calamities.
The most difficult thing is protection against fire and high temperature. Today, special heat-insulation materials withstanding high temperature are used. Such materials are costly and do not always provide sufficient protection against fire. The thing is that any heat-insulation material works passively, which means that it does not conduct thermal energy due to its low conductivity.

Instead of inventing new heat-insulation materials for a safe, Russian engineers decided to use one of the physical effects – phase transition. They directed their attention to ordinary water – a well-known and widely spread substance. When heated, water changes into steam absorbing much heat.
A fireproof safe has double walls with water poured into the cavity between the walls. In fire, the water absorbs heat, heats itself and changes into steam. The external wall of the safe has special valves for steam discharge.
This method of improving fire resistance is used in high-rise buildings. The frame of such buildings is made of hollow materials filled with water. 

Information source: “Izobretatel and Ratsionalizator” (“Inventor and Innovator”) , No.8, 2001.