Last year’s Chicago Cubs were an overwhelming team, riding one of the most impressive run differentials in recent history to a world championship. But this season, the Cubs have struggled to lift themselves above .500 despite playing in a mediocre division. Chicago should watch out — if it keeps playing this poorly, the Cubs will end up undergoing one of the largest drop-offs ever suffered by a World Series winner.

Teams often struggle after a championship. Even the most exceptional roster can get pushed back toward average through some combination of a World Series hangover and regression to the mean. But the Cubs’ slide is uncommon even by those standards. According to run differential, only five other champs in history have declined as much as Chicago has this year :

The Cubs’ decline is historic RUN DIFFERENTIAL TEAM YEAR CHAMPIONSHIP YEAR FOLLOWING YEAR DIFFERENCE Florida Marlins 1997 71 -256 -327 New York Yankees 1939 438 153 -285 Boston Red Sox 2013 197 -81 -278 Boston Red Sox 1912 268 23 -245 St. Louis Cardinals 1931 211 -34 -245 Chicago Cubs 2016 252 25 -227 Detroit Tigers 1935 271 53 -218 Los Angeles Angels 2002 207 -7 -214 Chicago White Sox 1917 199 14 -185 Pittsburgh Pirates 1909 265 83 -182 Cleveland Indians 1948 282 106 -176 All run differentials are based on a 162-game schedule. Source: Lahman’s Baseball Database

Of the 10 teams on the list, seven played before 1950, when baseball was a very different sport. In the current era (since 1988), the Cubs drop-off is the third worst, trailing only the 2013 Red Sox — more on them later — and the infamous fire-sale 1997 Marlins. Chicago had more room to fall than most champs — only 19 World Series winners matched the 2016 Cubs’ +252 run differential — but that just makes their current mediocrity stand out all the more starkly.

Boston’s decline between 2013 and 2014 provides the best recent precedent for Chicago’s slump. Just two years after current Cubs President Theo Epstein left the Sox, Boston won the World Series with a core roster that Epstein mostly assembled. The following year, however, the Red Sox disappointed their fans by scraping together a woeful 71-win season. This was part of a multi-season trend of the Red Sox zig-zagging between contention and mediocrity, and that stretch wound up being the most extreme set of year-to-year swings in MLB history.

But unlike the 2014 Red Sox or the ’98 Marlins, the 2017 Cubs have fallen apart while fielding a roster that’s largely unchanged from the year before. The Red Sox lost a star in Jacoby Ellsbury, along with an everyday catcher (Jarrod Saltalamacchia) and some bit players; the Marlins turned over nearly their entire team after winning it all. By contrast, Dexter Fowler counts as the Cubs’ only notable subtraction, and a healthy Kyle Schwarber (fresh off his World Series heroics) was supposed to offset Fowler’s loss. Instead, Schwarber floundered so much that he was was sent to the minors. (Granted, Fowler hasn’t been impressive with his new team, either.)

It’s unlikely that the Cubs will continue to be this bad for the rest of the year. Even with their poor play through the season’s first 83 games, most projections call for them to rack up many more wins over the second half. They ought to sneak into the playoffs in a division with few strong contenders, and once they’re there, anything can happen. But even if you credit Chicago with the elevated run differential that these projections expect over the rest of the season, rather than simply pro-rating their differential so far, the Cubs would still end the year with the eighth-largest decline in history. No matter what happens, the Cubs’ 2017 performance will have been just as historic as it was a year earlier, even if it’s not nearly so impressive.