$DDT$ is a chlorine containing pesticide used in agriculture. $DDT$ is rapidly passed through food chain causing biomagnification because it is stable, accumulates in the soil and concentrates in the fatty tissue or internal organs of body (as $DDT$ is fat soluble substance). So, $DDT$ takes may years to break down and it enters into body of other organisms with predation. So the amount of $DDT$ biomagnifies.