Customs officials in Bulgaria have arrested a driver carrying more than 3.4kg (7.5lb) of a rare metal that can be used in nuclear reactors.
The extremely pure sample of hafnium was found on a Bulgarian man trying to cross into Romania at Ruse.
There are no facilities to produce hafnium in Bulgaria, and it is illegal to import or export it from the country.
Police believe a trafficking gang was trying to smuggle the metal.
"The substance was transported as a metal, and in this condition it is not radioactive and is not dangerous," police said in a statement.
WHAT IS HAFNIUM?
Named after Latin for Copenhagen
Silvery 'transition metal'
Used in nuclear reactors
Used in Tungsten filaments
Hafnium has both civilian and military uses. It is used in the reactors of nuclear submarines and can also be used in rockets and bombs. But it can also be used for radio tubes and televisions.
Marina Nizamska, of the state Nuclear Regulatory Agency told French news agency AFP: "It is a very rare element - very beautiful and extremely expensive.
"The hafnium seized is almost 100% pure and the chances are that it was purposefully isolated to obtain that high level of purity. It is a double usage substance and police were right to confiscate it."
Police spokesman General Valentin Petrov told Bulgarian television that police suspect an international criminal gang was trying to smuggle the hafnium.