On 14 April 1945, there was a panic among the remaining German soldiers in Leeuwarden. The Canadians were already in Friesland and could enter the capital at any moment. The occupying forces were getting ready to retreat. But not without damaging important infrastructure in the city first.  

The post office in Leeuwarden was one of their key targets. The building was essential to the functioning of telephony and telegraphy in Friesland. Destruction of the post office would make communication between the advancing Canadians and the resistance sabotage groups much more difficult. German soldiers placed explosives in the basement of the building, ready to blow it up as soon as the enemy approached. Nearby residents were warned and had to leave their homes. But when members of the 'Sprengkommando' lit the fuse, nothing happened. The cubes of explosives had been replaced by fake copies made of wood. They had been fooled.   

Mark Wierda, an engineering student and intelligence officer in the resistance, was the mastermind behind this successful action. For weeks, he worked on his project. He meticulously created the fake cubes of 'trotyl', also known as TNT, used by the Germans. The cubes were weighted with lead to give them the right weight. Then yellow paper was pasted to them, but the colour of the paper turned out to be too dark. They were then bleached to the right shade of yellow under the Diaconessen Hospital's sun lamp installation. The original seal of the Trotyl manufacturer had to be forged as well. That too was a success and on 11 April 1945, the cubes were ready.  

Employees of the PTT carried out the last step of the plan: replacing the real explosives with the wooden cubes.  

When PTT technicians entered the basement of the building the next day, they found that the Germans had taken their anger out on the equipment with iron rods. Fortunately, the damage was limited. The monumental building on the Tweebaksmarkt had escaped disaster.  

Of the 72 cubes, seven remained undamaged. One was sent to Prince Bernhard, one ended up in the Postal Museum, and the remaining five are owned by the Fries Verzetsmuseum (Frisian Resistance museum).    

Mark Wierda did not live to see the result of his work. Together with his two younger brothers, he was executed near Dronrijp on 11 April 1945, four days before the liberation of Leeuwarden.