Polonium, the poison scientists suspect may be the cause of Palestinian leader Yasser Arafat's death in 2004, is a highly radioactive material rarely found outside military and scientific circles.
Swiss radiation experts have confirmed they found traces of polonium on clothing used by Arafat which "support the possibility" the veteran Palestinian leader was poisoned.
Arafat died in France on November 11 2004 at the age of 75, but doctors were unable to specify the cause of death. No autopsy was carried out at the time, in line with his widow's request.
His remains were exhumed in November 2012 and samples taken, partly to investigate whether he had been poisoned -- a suspicion that grew after the assassination of Russian ex-spy and Kremlin critic Alexander Litvinenko in 2006.
That investigation is ongoing, conducted separately by teams in France, Switzerland and Russia.
Also known as Radium F, it is a rare but naturally occurring metalloid found in uranium ores that emits highly hazardous alpha, or positively charged, particles.
Before being cited in connection with Arafat's death, the substance had been named in the case of Litvinenko, a former Russian spy and dissident who died in a London hospital in 2006 showing symptoms of polonium poisoning.
Small doses of polonium 210 exist in the soil and atmosphere, and even in the human body, but in high doses it is highly toxic if it is ingested or inhaled, and can damage the body's tissues and organs.
It is one of the rarest natural elements: in 10 grams of uranium there is a maximum of a billionth of a gram of polonium.
The substance has been used industrially for its alpha radiation in research and medicine, and as a heating source for space components, but in those forms it is not conducive to easy poisoning.
Small doses are also found in tobacco, derived from the soil and phosphate fertilizers used on tobacco plants.
Polonium was discovered by Marie and Pierre Curie in 1898 while they were doing research in France on the cause of radioactivity in the mineral pitchblende, the chief ore-mineral source of uranium.
Marie Curie named the material after her homeland of Poland, which at the time was under Russian, Prussian and Austrian control and not recognised as an independent country.
For their discovery of polonium, radium and their work on radioactivity, the Curies won the 1903 Nobel Prize in Physics, which they shared with Antoine Becquerel of France.
Like many other pioneering researchers in the field of radioactivity, Marie Curie died in 1934 at the age 67 of leukaemia, brought on by her handling of radioactive materials.