Experts have uncovered a series of grisly walls that are made of human bones at a cathedral in Belgium, the Brussels Times reports.

It's believed the structures were built in the 17th century and used bones that were already more than 200 years old. Included in the walls are thigh and shin bones from adults and several skulls, which have been shattered.

"This is a phenomenon we’ve not yet come across here," project leader Janiek De Gryse told the news outlet.

GRUESOME MYSTERY: 42 SKELETON 'WITH THEIR HANDS TIED BEHIND THEIR BACKS' DISCOVERED IN SHALLOW GRAVES

"For the moment we would place the actual construction in the 17th and 18th century," De Gryse added, noting there is "a great deal of research still to be done."

The gruesome discovery was found at Saint-Bavo’s cathedral in Ghent, Belgium, which has been used for more than 1,000 years, The Sun reports.

It's likely that the walls of bones were built when the church's graveyard was cleared in an effort to create new graves, with De Gryse stating that "skeletons cannot just be thrown away."

"Given that the faithful believed in a resurrection of the body, the bones were considered the most important part," he explained. "That is why stone houses were sometimes built against the walls of city graveyards: to house skulls and the long bones in what is called an ossuary."

Several other disturbing gravesites in Europe have been unearthed in recent months.

In January, four mysterious warrior graves from the 11th-century were discovered in Poland. Earlier this month, more than 40 skeletons with their hands tied behind their backs were discovered at a construction site in the U.K.

It's believed that the remains date back to Anglo-Saxon Britain, which lasted from the 5th to the 11th century A.D. or the bloody English Civil Wars between 1642 to 1651.

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Fox News' James Rogers contributed to this story.