Why do earth worms surface after it rains?

Worms need to remain moist in order to survive. The hot sun of the daytime will dry out their skin, and they will die, so they remain under ground and only come out during the moist night air to feed and move about on the surface. Worms breathe by absorbing oxygen through their skin and if there is sufficient oxygen in water, they will breath under water and will not drown, but rain water might not provide the most hospitable environment for their survival.

There are many scientific theories as to "why" worms come to the surface when it rains. Some research suggests that the underground water during rain can lack sufficient oxygen, which encourages the worms to seek the moist open air above ground. Another contributing factor is believed to be because the rain water makes the soil acidic, which can be fatal to worms, and perhaps the flooded tunnels and moist soil become too difficult to navigate. There is also the fact that worms move about and travel more easily on the moist surface which makes it easier to mingle with other worms and mate - which might be the primary reason they come out, or just a convenient benefit of the circumstances that drove them to the surface.