In metals processings such as welding, casting, and crystal growth the workpieces usually accompany with porosities, whose distributions, sizes and volume fractions determine the extent of any degradation in mechanical properties. Recently, the unsteady growth of a pore in solid behind the interface advancing in the upward direction has been experimentally and theoretically investigated. By accounting for unsteady mass, momentum, energy and species transport, and physico-chemical equilibrium on the cap surface, it has been found that the shape of the pore in solid is strongly affected by solidification rate. In earlier stage of solidification, rapid freezing enhances mass transfer and gas pressure in the pore. The growth of cap radius is attributed to the decreasing rate of gas pressure overriding that of hydrostatic pressure on the cap. As the decreasing rate of gas pressure becomes the same order of that of hydrostatic pressure in later time, the pore becomes a wormhole. This work shows that the pore formation in the solid is complicated but interesting processes including solute absorption from the atmospheric surroundings, convection in the pool, solidification, solute segregation, morphological stability, nucleation, detachment, growth and rise of bubbles, and mass transfer to bubbles. A review of them is provided in this work.