What irritated the local mayor most was the audacity of the tourists who tried to conceal their crime on the white-sanded Sinis beach along the west coast of Sardinia.

On a morning in late July, the visitors – a couple with a child, from mainland Italy – were spotted by a fellow beachgoer filling a plastic bottle with sand. The witness to the act immediately called the police.

“When the police arrived, the couple denied it and even tried to hide the bottle under a beach towel,” said Andrea Abis, who is the mayor of Cabras, a town and municipality that boasts 20 miles of pristine coastline. “It beggars belief. But unfortunately, this isn’t a rare occurrence.”

The couple were fined EUR1,000 (GBP850) on the spot.

Each summer, tons of sand are plundered from Sardinia’s beaches, despite it having been illegal since 2017 to take away sand, shells and pebbles from the Mediterranean island.

Fines range between EUR500 and EUR3,000, while those caught attempting to leave the island with significant quantities face jail terms.

The culprits, for the most part, are foreigners who cannot resist bringing home Sardinian sand as a keepsake, or for use in their fish tanks or to sell online.

Northern Europeans are particularly prone to filling up their camper vans with plastic bottles stuffed with sand, each one labelled with the name of the beach from where the precious grains were stolen.

In 2019, police snared a French couple with 40kg of sand contained in 14 large plastic bottles in the boot of their car, as they were about to board a ferry home.

“That has been the most mindblowing case to date,” said Carlo Lazzari, the group commander of Olbia financial police. “The couple wanted real Sardinian sand to decorate their aquarium.”

Lazzari is part of a special police unit that monitors the island’s three airports and ports. Every day during summer they find bottles of sand in luggage as travellers pass through baggage control. The team also scours the internet for illegal sales.

“There is a proper online market, and demand is high for sand from Sardinia. Most of the buyers are sand collectors,” said Lazzari.

Thousands of euros’ worth of fines have been issued so far this season. Lazzari said the majority of people did not realise they had committed an offence, and usually paid the fine swiftly so as to avoid legal repercussions.

An information campaign across TV, radio and social media has been stepped up this summer, highlighting the offence and how damaging it is to Sardinia’s environment. Vigilantes patrol beaches and signposts clearly warn that stealing natural resources is strictly forbidden.

“We don’t want to terrorise tourists, who for Sardinia are a resource, but to make people aware,” said Lazzari. “More than anything, our objective is to protect our environment.”

Over time, environmentalists in Italy fear the removal of sand could lead to the reduction of beaches, especially those such as Sinis, which contains white and pink-tinged quartz sand that is produced from the erosion of rocks.

“We call it fossil sand because, from a geological point of view, the grains don’t reproduce, and so every grain we lose we don’t get back,” said Abis.

In 2015, staff at airports and ports across Sardinia came together to create Sardinia Robbed and Plundered, a volunteer group that publicly denounces incidents of sand theft and spends most of the winter replenishing beaches from where the grains were taken.

“It’s a huge job but also very fulfilling as we know that we’re doing something advantageous for the environment,” said Franco Murro, the president of Sardinia Robbed and Plundered. “We Sardinians are very attached to our island.”

Murro said any removal of sand “creates a disturbance to the ecosystem and its dynamics”.

Among the most coveted grains in Sardinia are those found on Budelli, an uninhabited island off the north coast that is famous for Spiaggia Rosa, its pink-sanded beach. Before visitors were barred from the beach in the mid-1990s, sand theft was a regular occurrence.

Several people have regretted their transgressions and returned sand to Sardinia. On display at the mineralogical and wildlife museum in Caprera, an island close to Budelli, are samples of sand – in some instances the thefts date back to the 1980s – alongside letters of confession.

“When I went to the Caribbean, I came back with a few photos as a souvenir and nothing else,” said Tommaso Gamboni, a manager at the museum.

Gamboni and other volunteers monitor the beaches of the La Maddalena archipelago daily to ensure cleanliness and remind beachgoers about the rules. “But for us environmentalists, it sometimes feels like a lost battle,” he said. “We get a lot of insults whenever we try to educate people on the beach.”

The repenters are not enough to appease Abis. “Every time sand is removed, I feel as if a piece of my children’s future is being taken away. People are stealing something that can never be restored.”