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Why is Milwaukee City Hall sinking?

Many fungi, like these sulfur tuft mushrooms, thrive on damp, dead wood
Photo by Sean Gallup/Getty Images

Wooden pilings that support certain downtown Milwaukee buildings, including City Hall and the Milwaukee Repertory Theater, have recently been reported rotting. The rot is allegedly being caused by fluctuating groundwater levels and resulting in the buildings above slowly sinking. But reports have failed to explain the biological processes that link the alleged cause and effect.

Fungus is a leading actor among many natural processes and organisms that recycle dead wood back into an ecosystem. It includes a tremendous variety of species, ranging from yeast to mold to world-famous “humungous fungus," with a few key characteristics in common.

“Fungi are heterotrophic: They have to get their food from something else. They can't make their own food like plants do,” said Tom Volk, Professor of Biology at the University of Wisconsin-La Crosse. Volk, a mycologist, primarily researches fungi.

But Volk explained that unlike many familiar forms of life that digest food inside their bodies, fungi digest externally. If you wanted to eat your food like a fungus, said Volk, “you would dump your enzymes out onto it, and then you'd stick your hand into it and absorb the small molecules that have been digested.”

Fungi need water to live, but too much water interferes with these digestive enzymes. Flooded conditions also limit much-needed oxygen. Wooden pilings that are fully submerged in mud, as they were when the downtown buildings were constructed, consequently remain well-protected from wood-decaying fungi.

But when water-logged pilings become exposed to air, nearby fungi have an opportunity to eat and grow. Depending on the species, wood-decaying fungi eat either cellulose or lignin, both main components of wood, which together provide its structural integrity. A picture taken of a rotten piece of piling excavated from beneath the Milwaukee Repertory Theater shows the brown, crumbly remains typical of “brown rot”, when a fungus eats away the softer, white cellulose leaving the brown lignin behind.

Fungi are slow eaters, so the odds of the downtown structures being in imminent danger are slim, especially if the weight of a building is well-distributed on it's foundation. But the situation may become more precarious if the rotten pilings go unrepaired. “If the wood is rotting it becomes weakened, and in some places it's going to break,” said Volk. He added that even if the pilings don't break, the buildings might press down on them, resulting in more of the cracks and structural deformities that so far have only been measured in inches.