Thousands of tourists and visitors were evacuated from Paris’s world-famous Louvre museum midday on Monday amid an unknown security threat. Art lovers were told to leave the iconic institution and were ushered outside by staff.

Social media users uploaded photos depicting hundreds of people standing outside the museum next to I.M. Pei’s distinctive glass pyramid.

Details of the nature of the threat are scant. Kathy Skipper, who was affected by the evacuation, took to Twitter to write “Just evacuated from the Louvre. Just part of that Paris experience, I suppose.” Another visitor, Carmen Ng, in her Twitter post stated, “Inside Musee Louvre now and heard an alarm asking all visitors to evacuate as a ‘security protocol.’ What happened?”

In February, the museum was also evacuated after a man attacked a soldier standing guard outside the museum. The attacker was wielding a machete, but was immediately shot down by the soldier in question. According to the Express, former French Prime minster Bernard Cazeneuve described the attack as “terrorist in nature.” And in May, police again cleared the museum and the surrounding area amid a bomb scare on the day of the French election, after a suspicious piece of luggage was found by security.

French institutions in particular have been negatively affected by terrorism. Although the Louvre still topped The Art Newspaper’s annual museum attendance survey with 7.4 million visitors in 2016, it suffered a significant drop of 1.2 million visitors compared to the 8.6 million visitors recorded in 2015. Other Parisian institutions such as the Musée d’Orsay also suffered significant drops in visitor numbers, marking a decrease of 400,000 visitors from 3.4 million in 2015 to 3 million in 2016.

Falling tourism and rising security costs as a result of international terrorism is part of the new reality cultural institutions have to face.

The Louvre has yet to respond to artnet News’s request for comment.