Internal erosion of soil particles from within a dam by water that seeps through the dam is one of the most common causes of failure of levees and earth dams. 
Internal erosion is especially dangerous because there may be no external evidence, or only subtle evidence, that it is taking place. Usually a sand boil can be found, but the boil might be hidden under water. A dam may breach within a few hours after evidence of internal erosion becomes obvious.
Internal erosion manifests by the migration of soil particles by suffusion or piping.
Piping is induced by regressive erosion of particles from downstream and along the upstream line towards an outside environment until a continuous pipe is formed. 
Suffusion is the migration of soil particles through the soil matrix.