A piece of unknown debris floats just under the water in this image taken from a Royal New Zealand Air Force P3 Orion search aircraft while flying over the southern Indian Ocean looking for missing Malaysian Airlines flight MH370. Reuters

The day after Malaysia Airlines Flight MH370 mysteriously disappeared, on March 9, search teams spotted a “strange object” in the waters near Vietnam that they thought might have been a piece of the plane. It was the first possible lead. But investigators hoping for a trace of the missing Boeing 777 – a life raft, a seat cushion, a piece of the wing, anything – were disappointed to find only a mound of moss-covered trash.

As the search for Flight 370, presumed to have crashed in a remote region of the southern Indian Ocean with 239 people aboard, enters its fourth week, investigators have become increasingly frustrated as time and time again their leads turn out to be dead ends. While officials have narrowed their search range to a 200,000-square-mile area of water roughly 1,150 miles west of Perth, Australia, most of the floating debris they have encountered has had nothing to do with the missing plane.

The problem? Our oceans are chock-full of junk.

"A lot of the stuff we are seeing is basically rubbish,” New Zealand airman Andy Scott, who has been part of the search crew looking for signs of the downed plane, told the Associated Press. He said his crew aboard the P-3 Orion search aircraft spotted about 70 objects within four hours on Saturday.

Australian surveillance planes scoured the ocean near Perth over the weekend and spotted four orange objects in the water, each about 6 feet in length. It was a promising lead, but every object ships recovered turned out to be fishing equipment and other trash.

CNN correspondent Will Ripley said Sunday that leads like this are “promising from the air,” but “when you actually get down to the ocean and you recover the items, it might turn out to be a different story. That seems to be the case that we are now learning.”

So just how much rubbish do our waters contain? It is estimated that every year, 14 billion pounds of garbage are discarded into Earth’s oceans. Tires, fishing nets, polystyrene buoys and plastics of all kind end up in the seas, floating just below the surface or breaking up to form a kind of junk soup.

According to CNN, ships dump 5.5 million pieces of trash into the sea every day. There was the time a cargo ship accidentally spilled 2,000 computer monitors into the water, and another instance in which thousands of pairs of Nike sneakers were inadvertently dropped.

Most of the garbage that ends up in our oceans is plastic. Much of the debris collects in five ocean gyres, which are circular vortexes where ocean currents formed by Earth’s wind patterns and the planet’s rotation draw garbage in and trap it in perpetually spinning islands of debris.

One of these vortexes is located in the Indian Ocean west of Australia.

“Any search and rescue attempt will be hampered by untold quantities of debris,” Los Angeles captain Charles Moore, an environmental advocate and one of the first people to call attention to the giant patch of garbage in the north Pacific Ocean known as the Great Pacific Garbage Patch, told the Associated Press. “It’s like a toilet bowl that swirls but doesn’t flush.”