Mystery Polymarket traders raked in huge profits after correctly predicting an unusual temperature spike at a weather station in Paris-Charles de Gaulle Airport, prompting French officials to launch an investigation into suspected tampering.

Polymarket, a leading prediction market, allows users to place bets on the maximum temperature based on readings at specific locations around the world. On April 15, the temperature in Paris was 18 degrees Celsius and trending downward when it inexplicably spiked to 22 degrees Celsius at 9:30 p.m. – before rapidly plunging again.

Just before the spike, Polymarket user “xX25Xx” placed an approximately $120 bet that the day’s top temperature in Paris would exceed 18 degrees Celsius – even as 99% of other users predicted it wouldn’t, the Wall Street Journal reported, citing data from analytics firm Bubblemaps. The user earned more than $21,000 in profit on the trade.

A similar incident reportedly occurred on April 6, when a Polymarket user called “Hoaqin” earned nearly $14,000 after predicting that the temperature in Paris, which had peaked at 18 degrees Celsius late that afternoon, would hit 21 degrees.

Météo-France, the country’s weather forecasting office, examined its sensors at the airport and ultimately filed a complaint with local police due to concerns that someone had meddled with the system, a spokesperson told the Journal.

Tampering with temperature sensors in airports is potentially dangerous because airlines and air traffic controllers rely on accurate data to safely operate flights.

Polymarket representatives did not immediately answer a request for comment.

Since the wagers, the prediction market has switched to gathering its weather data for Paris at a location in Paris-Le Bourget Airport, according to its website.

Polymarket and Kalshi have faced growing scrutiny in recent months due to concerns that users could attempt to rig “prediction” bets in their favor.

Recent incidents include a wave of bets on Polymarket that correctly predicted when the US would begin airstrikes on Iran, as well as a surge in trading activity on oil futures on March 23 just minutes before President Trump announced he wouldn’t target Iran’s power plants.

Earlier this month, the White House warned staffers not to use any inside information to place bets.