According to reports, over 2,000 World War II weapons and other items have been excavated.
Military spokesman Tomasz Znojkiewicz said: “We are dealing with a stockpile of weapons that are more dangerous now than 70 years ago.”
Construction workers came across the objects while digging up a road, and they were initially described as “unexploded ordnance”.
The operation was also temporarily interrupted earlier by the discovery of human remains at the same site.
It is believed that the stockpile was abandoned by Germans withdrawing from Silesia at the end of the war.
Mr Znojkiewicz added: “So far, 1,081 bombs have been excavated and secured, but I believe that it is only half of the entire arsenal buried here.”
Two nearby homes were evacuated, but residents have now been allowed to return.
They were provided with a hotel for several nights.
According to reports, the bombs were safely detonated on a nearby field.
Mikolow mayor Stanislaw Piechula said: “German forces frequently abandoned weapons in Mikolow towards the end of the war.”
In addition to weapons, authorities also found German helmets, gas masks and bottles. In total, there are over 2,000 items.
Experts will analyse the stockpile and identify interesting items that can be displayed in museums.