Osmosis is a physical process in which a liquid or fluid passes through a semipermeable membrane through its pores when pressure is applied.
The problem of osmosis in fiberglass swimming pools is very common. The gel coat, which is the glossy and initially waterproof material that covers the structure, over time (not long) loses its properties and becomes permeable and microporous, allowing moisture to pass from one side of the pool to the other.
When the gel coat coating becomes microporous, it allows both the liquid inside the pool shell, i.e. the water, to pass through the structure and out, causing water leaks.
Also, in the opposite direction, environmental moisture present in the earth surrounding the fiberglass pool, can pass through the structure and accumulate in the micropores it now has, rotting and forming bubbles visible from the outside and black spots, which when when they explode they emit a very unpleasant black liquid.