The tidal forces, a result of the gravitational interaction between Mars and Phobos, are so great that the planet is slowly tearing apart its moon. The distance between the moon and Mars is already decreasing by almost two centimetres every year.
This same phenomenon also occurs between the earth and the moon, but, unlike with Phobos, there is no danger that our moon could break under these forces.
In the case of Phobos, astronomers assume that the moon will break up in about 50 million years. Some of its debris will then crash onto Mars, while smaller fragments and dust will collect in a low orbit and eventually form rings similar to those surrounding Saturn.