The Marfa lights or the Marfa Mystery Lights are unexplained lights (known as "ghost lights") usually seen near U.S. Route 67 on Mitchell Flat east of Marfa, Texas of the United States.

The first confirmed report of the Marfa Lights dates to a 1957 magazine article. Earlier oral reports of the Marfa Lights have been attributed to Robert Reed Ellison in 1883. According to the historian Cecilia Thompson, writing in 'History of Marfa and Presidio County', Mr Ellison did not record the Marfa Lights in his memoirs written in 1937 but told his family about them and this information was orally relayed by them. No verifiable written sources for the Marfa Lights predate the 1950's.

Reports of evidently similar strange nocturnal lights in this area have persisted all through the twentieth century, and they continue today. These reports often describe brightly glowing balls floating above the ground, or sometimes high in the air. Colors are usually described as white, yellow, orange or red, but green and blue are sometimes reported. The balls are said to hover at about shoulder height, or to move laterally at low speeds, or sometimes to shoot around rapidly in any direction. They often appear in pairs or groups, according to reports, to divide into pairs or merge together, to disappear and reappear, and sometimes to move in seemingly regular patterns. Their sizes are typically said to resemble soccer balls or basketballs.

The Marfa lights are elusive but not genuinely rare. They appear occasionally and unpredictably, perhaps once in about two weeks, averaged over time. There are no reliable reports of daytime sightings; the lights seem to be a nocturnal phenomenon only. They may appear at any time of night, typically south of U.S. Route 90, five to fifteen miles east of Marfa, at unpredictable directions and apparent distances. They can persist from a fraction of a second to several hours. There is evidently no connection between appearances of the Marfa lights and anything else besides nighttime hours. They appear in all seasons of the year and in any weather, seemingly uninfluenced by such factors. They sometimes have been observed during late dusk and early dawn, when the landscape is dimly illuminated.

It is extremely difficult to approach an ongoing display of the Marfa lights, mainly due to the dangerous terrain of Mitchell Flat. Also, all of the land where the Marfa Lights are observed is private property, and access is prohibited without explicit permission from the owners. There are only a very few accounts of success in moving very close to observed lights, but those that exist generally describe objects resembling fireworks lacking both smoke and sound.

Less frequent accounts of seemingly similar anomalous nocturnal lights have arisen along a broad and elongated region within west Texas, stretching generally from El Paso southeastward along the Rio Grande valley, past Big Bend National Park and farther southeastward into Mexico. Also, repeated appearances of apparently similar lights have been reported worldwide. Some of these emerge, and then seem to fade over time, and finally disappear. Others persist over many years. Undoubtedly the most renowned among the latter are the Hessdalen lights, of Hessdalen, Norway. A similar, less well recognized, persistent phenomenon are the Min Min lights of northeastern Australia, and a number of other like cases are known. In the Gurdon, Arkansas area, there is a single light there that has bizarre properties. It has been featured in local media nearly every Halloween and on the show Unsolved Mysteries. This is called the Gurdon Light.

Skeptics discount paranormal sources for the lights, attributing them to mistaken sightings of ordinary nighttime lights, such as distant vehicle lights, ranch lights, or astronomical objects. A few suggest they have deliberately been given a paranomal mystique designed to attract tourist business to this remote west Texas area, pointing out that it wasn't until July 1957 that the earliest published account of the Marfa lights, "The Mystery of the Texas Ghost Light," by Paul Moran, appeared in Coronet Magazine. These critics challenge the historical reports cited by this and other published accounts as having been manufactured.

A number of projects carried out by nonresident investigators over several decades have generally confirmed the appearance of the anomalous lights often with photographic and video evidence. Many suggestions have been offered to explain the reported observations, but no consensus has been reached.

The dominant skeptical explanation seems to be that the lights are a sort of mirage caused by sharp temperature gradients between cold and warm layers of air. Marfa is located at an altitude of 4,688 feet above sea level, and temperature differentials of 50-60 degrees between high and low temperature are quite common. Proponents of this explanation reject the close-range accounts of the phenomenon, which they regard as invariably anecdotal.

Some contended that the lights are the result of a naturally occurring phenomenon, the piezoelectric effect, discovered by Pierre Curie in 1883. In this case, critics contend that the mountainous region is made up of mostly rocks containing quartz that expand during the day and contract at night, due to thermal expansion. This expansion and contraction creates stress on the quartz crystals which in turn is converted into voltage that is accumulated over time until it is then discharged into the atmosphere creating a ball lightning effect.

In May 2004, a group from The Society of Physics Students at the University of Texas, Dallas spent four days investigating and recording the Marfa lights. Their report  proved the following points.
U.S. Highway 67 is visible from the Marfa Lights viewing location
The frequency of the Marfa Lights correlates with the frequency of vehicle traffic on U.S. 67
The motion of the mystery lights behaves in a predictable fashion
At least one mystery light was directly correlated with a vehicle on U.S. 67 observed by a chase vehicle.

They came to the conclusion that all of the mystery lights observed could be reliably attributed to automobile headlights traveling along U.S. 67 between Marfa and Presidio, TX.

Note that an independent discovery, http://godshome.us/marfa, reveals another example of how ordinary automobile headlights are visible from great distances.