In the engine room of all Navy ships, there are sea water heat exchangers that must be hydro lanced periodically for cleaning and maintenance. This involves months dead sea life, rotting in thousands of small tubes. The person doing the maintenance must crawl inside the heat exchanger itself. Often times the hydro lance is not enough, and pulling a knotted rope through the tubes is necessary. There are a lot of dirty jobs in the Navy, but this is one of the dirtiest.