To assess the extent of the impact, Fortune collected data from leading prediction markets as of Thursday. Kalshi showed the largest increase: the platform reports that World Cup trading volume has reached $2.9 billion, including combination bets, and continues to rise. That figure has already exceeded other marquee sports events, including March Madness, which saw the firm bring in $2.51 billion, and this season’s Champions League, where bets totalled $685 million.

Polymarket, a major rival to Kalshi in the space, has reported similarly strong activity. According to a person familiar with the company, Polymarket’s World Cup wagers have generated $2.5 billion in cumulative trading volume since launching last July, making it one of the largest single markets in the platform’s history. Over the same period, soccer-related trading on its global decentralized finance platform has exceeded $5 billion.

Robinhood, for its part, shared World Cup prediction market activity by citing contract volume. A spokesperson told Fortune that Rothera, a CFTC-licensed exchange and clearinghouse operated through the platform’s joint venture with Susquehanna International Group, has executed more than 500 million contracts since the start of the month, including 400 million since June 11, the tournament’s opening day.

Prediction markets are not just having a World Cup “moment.” Instead, this tournament has helped confirm their rise in the United States, where they are increasingly used as an accessible way to bet on just about everything. These platforms let users trade small-dollar positions on real-world outcomes from their phones. For many fans, backing a team or scenario in a market now sits alongside traditional sportsbooks and fantasy leagues as a default way to participate in big events.

Given the size of these markets and how unpredictable soccer can be, individual bets can end up either very profitable or very costly.

Windfalls and wipeouts

Many of the World Cup matches so far have taken unexpected turns. Spain, one of the tournament favorites, drew 0–0 with Cabo Verde in its initial match last Monday. Cristiano Ronaldo’s Portugal also recorded a surprising draw against Congo, a team widely viewed as a straightforward win on paper.

In this kind of unpredictable environment, putting money on the line can lead to either significant gains or steep losses for bettors. Since the start of the World Cup, both outcomes have already played out.

In the Portugal–Congo match, a Polymarket user with the handle “BreakTheBank” wagered just under $300,000 that Portugal would not win and ultimately booked a profit of nearly $1 million when the match ended. In another instance, a newly created wallet placed a $4 million bet on the same platform that Spain would not beat Cabo Verde and walked away with roughly $9 million, prompting public questions about whether the trade reflected good luck or access to nonpublic information.

These kinds of trades have intensified concerns about insider activity on prediction platforms. Some countries have moved to ban or block certain markets entirely, and in the United States, the rapid growth of event contracts has sparked an ongoing fight between federal regulators and state authorities over whether these products are closer to financial instruments or unlicensed gambling.