Some coastal habitats, namely estuaries, receive large quantities of pollutants discharged by rivers opening into the sea. Other habitats, such as mangroves, are being drained and developed for agriculture or for human settlement. Salt marshes are considered as unhealthy places where mosquitoes breed, therefore, man has drained salt marshes over the years. Examples of drained salt marshes in the Maltese Islands include: Marsa which was drained by the British; and Salina which was drained by the Knights of St. John.