The monuments were covered by the skull masks, which were topped with wreaths of branches, and many of the skulls had an ‘X’ made from black tape covering their mouths.
This led many people to believe the skulls might have been a protest against the silencing of the anti-vax movement.
The mystery has since been solved, with the Italian branch of Extinction Rebellion owning up to the stunt.
In a statement released via Twitter, the Pisa chapter of Extinction Rebellion said that the skulls were part of an installation by artist Michele Tombolini.
“The skull symbolises the death of living species because of the climate collapse," the tweet read.
"The plant represents life and the salvation that is still possible, but only if we act immediately.
“The 'X' appears on the mouth of the skulls because the story of the emergency is still not being told.”
The artist acknowledged the importance of peaceful protests, noting that none of the monuments had been damaged by his installations.
“This artistic intervention stems from the desire to help raise awareness for the environmental issues that are increasingly putting a strain on the life of the planet,” he continued.
“I am aware that when working with public objects this must be done with the utmost respect for them, without causing any damage.”
