Sometimes all the sewage does not get treated properly at the treatment plant before entering the ocean. This is because it doesn't get to the treatment plant at all. Storm drains, which are connected to the sewer pipes, allow rain water to be piped out directly into the ocean. However, sometimes in the case of heavy flooding, the amount of water being drained away is less than the amount of water coming in, so that an overflow occurs. Rain water and untreated sewage mix, where it then flows right into the ocean, causing harmful pollution.