## 2 Underwater Explosion  
An explosion can be defined as a rapid increase in volume, and a release of kinetic energy or potential energy. The explosion generates a blast pressure wave or shock wave, high temperature and release of gases, in conjunction with loud and sharp sounds caused by the incidents that are associated with the occurrence of each explosion phenomena [7; 8; 9]. It is also a process of rapid burning with increasing pressure occurring in fractions of a millisecond [10; 11; 12; 13]. The nature of this process is determined by the dynamic conditions in which the flammable mixture is found, in particular the turbulence of the medium. Pressure waves called the shock wave arising during an explosion in liquids (underwater explosion) or in solids reach up to 8000 m/s in the case of detonation. It is also worth differentiating between other processes that may occur during the initiation of an explosive charge.  
Due to the speed of the reaction, a distinction is made between combustion, deflagration and detonation. Normal combustion is a process with a propagation velocity of the order of mm/s, for deflagration they are m/s, and for detonation--km/s [3; 14]. After reaching the propagation speed \(>\) 1000 m/s, deflagration goes into the so-called low order detonation, and at speeds \(>\) 5000 m/s is referred to as high order detonation [15]. The transition of deflagration to detonation is accompanied by a change in mechanism: from a process driven by heat to driven by a shock wave [16]. Deflagration occurs on the surface and in a uniform manner, its driving force is generated heat, and the speed of the process depends on the thermal conductivity of the material [17; 18]. The direction of propagation of reaction products is opposite to the direction of reaction propagation (these directions are consistent during detonation). Deflagration of the explosive occurs much more rapidly than the combustion of ordinary combustible material, accompanied by a flame or sparks and clear sound effects. It is the basic explosive transformation of propellants (gumpowder). It can run without air [14; 15; 16].  
The product of the underwater explosion is also overpressure, which, depending on the mass of the load reaches a value of 1 to 10,000 MPa. Before the arrival of the shock wave front, the pressure is equal to the hydrostatic pressure of the liquid. With the arrival of the wave front, the pressure increases rapidly up to a maximum value, called peak positive overpressure. The pressure then drops to its original pressure (Figure 1). The period of further pressure drop and its return to hydrostatic pressure is called the negative phase period. Important parameters of the whole process are the maximum value of overpressure and the area under the function describing the pressure dependence on time during the positive phase. The nature and mechanism of the explosion decide many parameters, which include [19]:  
* Material properties (physical, chemical, stability, heat of combustion, etc.);
* Space where combustion occurs (size, open, closed, obstacles, etc.);
* Properties of the explosive mixture (concentration, pressure and temperature);
* Ignition method (energy, temperature).