Sept. 11, 2015: The abandoned building of a German nazi-era railway installation in Walim, near Walbrzych, in Poland. (AP)

A Polish explorer said on Friday he found a massive underground structure in southwestern Poland that the Nazis supposedly built to protect thousands of people.

Krzysztof Szpakowski told a news conference that he has no doubt that he found a Nazi shelter. He said his claim was based on evidence that he gathered for decades, including witness statements, old documents and an examination of the area by ground-penetrating radar and by dowsers.

Szpakowski told Polish television station TVN24 that the structure was part of the “Riese” system of railway tunnels, corridors and shelters that the Nazis were building during World War II in the mountains around the city of Walbrzych.

He claims anything from technical appliances to armaments could be inside, but “not a gold train” that has been sought after two other treasure hunters claimed they found last month.

Two other men have sparked a gold rush in Poland after they claimed to have found a tunnel at a separate location in Walbryzch that contains a Nazi train said to be carrying valuables. Now they say it could be rather precious minerals for the war industry. However, nothing has been found yet.

The Guardian reports the train’s existence has yet to be proven. However, a deputy culture minister told the newspaper last month he was “more than 99 percent” sure it was true.

Historians say there are dozens of miles of tunnels and only 10 percent of them have been uncovered. No documents have been found to indicate their overall purpose, but they have attracted explorers and inspired legends of a hidden Nazi train laden with gold.

The region was in Germany until the war's end and then became part of Poland. The Red Army resided in the area for some 18 months after the war before handing it over to the Poles.

The Associated Press contributed to this report.