Several underground bunkers — left from WWII — unearthed at park in Belgium. See them

Along the coast of Belgium, a team working to clean up a park encountered much more than just invasive plants.

The sandy dunes hid several long-forgotten secrets.

The nature restoration project at a park in Knokke-Heist planned to remove some weeds and restore some dunes, the Municipality of Knokke-Heist said in an April 3 news release.

But as the team dug into the sandy ground, they quickly hit a large concrete structure.

The workers had stumbled on three underground bunkers left from World War II, archaeologists said.

The hidden bunkers were made of reinforced concrete about 3 feet thick and still completely intact. The single-room structures were accessed from an entrance in the roof, officials said. A photo shows the slightly run-down interior.

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During WWII, the Nazi German Army built a military stronghold at the Directeur-Generaal Willemspark in Knokke-Heist, officials said. The site, known as Stützpunkt Heyst, was considered impenetrable.

The Nazi German Army built roughly 60 defensive structures along this stretch of coastline between 1942 and 1943, the city said. The fortifications included barriers, walls, ammunition bunkers, military quarters and other structures all intended to deter an invasion from the sea.

After the war ended, many of these fortifications were demolished or buried, officials said.

The recently rediscovered trio of underground bunkers are just the tip of the iceberg, archaeologist Sam De Decker told VRT NWS, a Belgian news outlet. Other larger structures from WWII are likely still buried under the modern-day park.

Excavations at the park also found some brick trenches, walls, a well, ammunition and other rubble dating from WWII, the city said.

Officials carefully documented the bunkers, then reburied the structures to preserve them.

Knokke-Heist is along the northwestern coast of Belgium and about a 70 mile-drive northwest of Brussels.

Google Translate was used to translate the news release from the Municipality of Knokke-Heist and article from VRT NWS.

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