The Bermuda Triangle is a patch of the Atlantic Ocean, out at sea between Florida, Puerto Rico and the island of Bermuda. The name comes from a 1964 magazine article by a writer named Vincent Gaddis, and for many years people have told stories and written books about it. Many of those stories mention compasses, so it helps to know what a compass does. A compass needle follows Earth's magnetism, which helps people tell which way is north. More exactly, a compass needle points along the horizontal part of the magnetic field where it is, not toward any single point such as the magnetic pole. A compass shows the direction of the field in the place where it sits, and that is the first thing to keep in mind.

Two kinds of north are in play. Magnetic declination, also called magnetic variation, is the angle between magnetic north, where a compass points, and true north, the direction of the North Pole. Magnetic north is where the needle points, and true north is the direction of the pole. They are not the same direction. Magnetic declination is different from place to place and slowly changes over time, because magnetic north keeps shifting. The angle between the two kinds of north is therefore a number that belongs to one place and one time.

For a navigator, declination is a number to look up. To get a true bearing from a compass bearing, a navigator adds the magnetic declination for that place, so forgetting it would send a ship or plane off course. Because declination differs from place to place and changes slowly over time, the number for one place and year is not the number for another.

Compass stories about this ocean are old. Christopher Columbus wrote in 1492 that his compass readings were odd while he sailed through this part of the Atlantic. Some writers say true north and magnetic north lined up in part of the Bermuda Triangle long ago, which could have thrown off sailors' and pilots' navigation. NOAA says there is some evidence that in the Bermuda Triangle a magnetic compass sometimes points toward true north instead of magnetic north. All three statements are about one idea: where a compass points can differ from what a navigator expects.

In the 1940s there was no help from the sky. The first weather satellite was launched in 1960, so in the 1940s nobody could watch storms from space. Before weather forecasts got better, ships in the area had less warning about storms than ships have today. Ships and planes found their way with compasses, charts and radio direction finders, among other tools. In December 1945 a flight of five Navy training planes, known as Flight 19, lost track of where it was over the Atlantic off Florida. Its leader reported that the compasses seemed to be malfunctioning. The Navy's record said the Flight 19 planes were lost for 'causes or reasons unknown'. Those words are the Navy's own, and they say that the record did not name a cause.

In the 1970s a librarian named Larry Kusche checked the original records behind the Bermuda Triangle stories instead of just retelling them. Kusche used Coast Guard records, weather records, maritime records at Lloyd's of London and newspaper stories from the time instead of hearsay. He concluded that the Flight 19 training flight was lost mainly because the pilots lacked experience; he spent seven years researching it, including interviews and retracing the route. Larry Kusche also found that ships said to be lost in calm weather had actually met storms. His answer on Flight 19 came from records, interviews and retracing the route.

Kusche also looked at how the stories were told. He concluded that a typical writer retold stories from earlier books and added details, so the stories grew like a secret whispered around a circle of friends. Once the legend was established, reporters were quick to blame more wrecks on the Triangle. The frenzy has since eased, perhaps because modern technology tracks ocean and air traffic more precisely. A story passed from book to book can gain details that no record holds.

Official agencies have given their view too. NOAA, the U.S. ocean science agency, says there is no evidence that unexplained trouble happens more often in the Bermuda Triangle than in other busy parts of the ocean. The U.S. Navy and U.S. Coast Guard say there are no supernatural explanations for disasters at sea, and that nature and human mistakes together explain them. The Coast Guard has said that a review of many aircraft and vessel losses in the area found nothing that indicated anything other than physical causes, and that no extraordinary factors have ever been identified. So the question for any story about a lost flight is simple: what did the flight know about where it was, and how did it find out?

Words to know

• declination: north-angle gap
• magnetism: pull of magnets
• navigator: route finder
• hearsay: passed-on talk
• malfunctioning: not working right