Sea Foam

Ever experience sea foam?????
Not like this you haven't.

http://www.dailymail.co.uk/pages/live/articles/news/worldnews.html?in_article_id=478041&in_page_id=1811

A skimmer for a salt water aquarium (also sewage treatment plants) uses the same principle.  Small air bubbles are injected into the bottom of the skimmer, travels up through the water column in the skimmer where impurities attach themselves, and forms contaminated foam in the collection cup at the top.     

Compass definitely needed at that point.