Two-thirds of the planet is covered with water, which makes finding something lost at sea an imposing task.

Four days after Malayasia Airlines Flight 370 disappeared enroute to Bejing, search and rescue vessels scouring the region have found no trace of the airliner or the 239 people aboard. Although authorities have yet to speculate on what happened aboard the Boeing 777-200, what ever it was that brought down the plane is widely believed to have occurred quickly, catastrophically and at high altitude. That would scatter debris over a huge area.

You'd think that would make finding debris easy, but that has not been the case. Malaysia Airlines says nine aircraft and 24 ships are searching for Flight 370; the flotilla includes the USS Kidd and USS Pinckney, two destroyers that were conducting exercises in the area. The U.S. Navy also deployed a Lockheed P-3C Orion, a maritime surveillance plane originally developed for anti-submarine work. This search force, drawn from nine countries, has expanded its focus to a vast swath of the South China and Andaman seas, the Straits of Malacca and the Gulf of Thailand–an area larger than Texas and California combined.

Despite the size of the search operation and the technology at its disposal, the task of searching for an aircraft in the water still comes down to sailors and airmen looking at the sea, for hours at a time.

"Finding pieces and parts from the air is very difficult to do," said retired Coast Guard Lt. Commander Larry Kidd. Although a portion of the search effort is focusing on land masses in the area, that doesn't make the task any easier. There are some remote areas in that part of the world, and "they could lose an airplane or pieces of an airplane and never find it," Kidd said.

Large-scale pelagic search-and-rescue operations are managed from what's called a Rescue Coordination Center. Officials there coordinate the efforts of the various nations and agencies involved, ensuring efforts are not duplicated and the area in question is thoroughly and efficiently searched. Because Flight 370 was from a Malaysian carrier, departed from Malaysia, and presumably went down relatively close to home, that country's Department of Civil Aviation is running the show. Malaysian authorities have considerable experience with search and rescue operations, and the country's expertise is well regarded by others in the field.

The biggest challenge, aside from the size of the search area, is not knowing where Capt. Zaharie Ahmad Shaw and First Officer Fariq Ab Hamid ran into trouble, where the plane went down, or why. Knowing where to begin the search is, of course, a key data point, said Petty Officer 3rd Class Katelyn Shearer of the United States Coast Guard. Although she would not speak specifically about the search for Flight 370, she outlined what typically happens in such a search and rescue operation.

When the call comes in–either a distress call from an aircraft or a ship, or an alert from another agency–authorities direct any available ships, helicopters and aircraft toward the vessel's last known position. The number of vessels deployed will depend upon the situation–an aircraft carrier wouldn't respond to a sailboat sinking a mile offshore, for example, but would be dispatched if it were the vessel closest to a distressed ship on the high sea.

If the vessel in distress cannot be promptly located, search and rescue craft begin a search pattern. The Coast Guard has five general patterns, and which one is deployed depends upon the accuracy of any information about where the distress call was made and whether, and where, datum–possible debris sightings–are reported. The pattern used is determined by the current, wind and other factors and also the type of vessels involved in the search. Using a specific pattern ensures the search is conducted efficiently and accurately.

"Search patterns are valuable because they allow crew members to complete a thorough and a methodical search of the area," Shearer said. Depending on what's available, search patterns will be done via air and sea, with the coordinating authority assigning tasks to individual vessels to ensure coverage. Each vessel has a specific advantage–and drawback. An airplane can survey a much larger area than a ship, and do so in much less time, but it is not as useful for locating or investigating small debris.

One of the biggest issues in searching for anything at sea is dealing with current and drift. Computer models and meteorological data help here. The Coast Guard, for example, uses computer models to "determine the most appropriate search pattern based on currents, wind and other external factors," said Shearer. The Coast Guard software can also predict in which direction the vessel may have drifted, but as time passes, predicting drift becomes increasingly difficult.

Members of the Chinese emergency response team salvage a floating object at the possible crash site of missing Malaysia Airlines flight. Photo: © Zhao Yingquan/Xinhua Press/Corbis

In the case of Flight 370, rescue planes would have started with a track line search, tracing the flight path of the plane from the point of departure in Kuala Lampur all the way to Beijing, said Kidd. Then, search assets would begin adjusting for lateral drift, performing what's called a parallel track search. Kidd said airplanes would likely fly in five-mile-wide patterns, with ships searching one mile at a time. If something was spotted, a radio beacon typically is dropped into the water and new search grids developed based on that location.

Even with all the technology in the world, search and rescue operations come down to men and women scanning the sea with binoculars. This is an exceedingly daunting task when you consider the search area covers as many as 500,000 square miles. And this explains how it is possible not to have found any debris yet. Searching open water is slow, tedious work, made more so by the time required to get anywhere.

With something like a missing commercial airliner, help from other, less traditional sources, comes into play. The U.S Government reviewed imagery taken by its spy satellites in the region for possible evidence of an explosion, to no avail, according to Reuters. DigitalGlobe, a commercial satellite imagery firm, has taken numerous pictures of the search area with its five satellites, posting the pictures on its website in an attempt to crowdsource the search.

The odds of finding survivors dwindles over time, but U.S. Navy and other vessels involved with the search are equipped with medical facilities should survivors be located, and hospitals are on alert.

Kidd says that if the plane had hit the water in one piece, there would most likely would be a lot of debris in a concentrated area. However, if it broke up at altitude, pieces could be scattered far and wide, making it hard to identify any single item from the air. This, combined with the challenge of seeing debris of any kind in the water, can make finding things difficult. Early in the search it was widely reported that an airplane door had been spotted, but authorities didn't conclusively identify what it was and it hasn't been spotted again. What was thought to be debris in another area turned out to be a coral reef.

And so the search goes on, for as long as it takes. It took two years before authorities found the black box data recorder and the airframe for Air France Flight 447, which went down over the Atlantic in 2009. But at some point, the search will scale back. When and how that happens will be up to officials at the Rescue Coordination Center.