The Pittsburgh Pirates haven't had a winning season since Bill Clinton's first term in the White House.

The streak of 20 years without the team ending a single season over .500 is, as Pirates fans hate to be reminded, the longest such drought in the history of professional American sports.

Yet this year, with just over half of the season gone, the Pirates have the second-best record in all of baseball as of Tuesday night. At 18 games over .500, the Pirates would need to go 27-47 over the rest of the season to finish with a losing record for the 21st year in a row.

How did the Pirates, a team that lost 105 games just three years ago, suddenly become one of the game's most dominant teams?

The answer has nothing to do with batting.

The Pirates rank 24th out of 30 teams in runs scored, and 23rd in batting average (only center fielder Andrew McCutchen has a batting average over .300.) The team's .310 on-base percentage ranks 22nd in baseball, placing them between the Seattle Mariners and San Diego Padres, both of which have 13 fewer wins than Pittsburgh does this year.

While the team's hitting has been remarkably subpar, their superb pitching has kept them afloat. The team ranks first in baseball with an incredible 3.15 ERA; they lead the league with 12 shutouts; and they're third in WHIP (walks plus hits per inning pitched).

The pitching staff has been anchored by some surprising performances. Twenty-five year-old Jeff Locke, in his first full season in the majors, ranks second among all starters in the game with a 2.15 ERA. That puts him just ahead of the Mets' much-hyped phenom, Matt Harvey.

A couple of reclamation projects have also pitched well beyond their expected levels of production this year.

The Yankees paid Pittsburgh $13 million to take A.J. Burnett off their hands in 2012 after the pitcher scuffed through three miserable years in the Bronx — he posted a fat 5.15 ERA in his final year in New York. Burnett turned things around immediately upon arriving in Pittsburgh, putting up a 3.51 ERA last year. He's now on pace to post the lowest ERA and WHIP of his entire 15-year career.

Similarly, the Red Sox swapped the underachieving Mark Melancon for former Pirates closer Joel Hanrahan before the 2013 season. Hanrahan is now done for the year with an elbow injury. Melancon, who became a pariah in Boston with a 6.20 ERA last year, has a microscopic 0.85 ERA this season, and has been one of the best relievers in all of baseball.

Given that fantastic run suppression, the Pirates as a whole boast a +41 run differential, sixth best in the game. Statheads love to point out that run differential is a highly accurate predictor of a team's success. Based on that, then, it's clear the Pirates' success is no fluke.

Fangraphs' Dave Cameron, who wrote at the end of May that it was "time to take the Pirates seriously," wrote again last week that, despite his belief that the team would regress slightly as the year wore on, they had actually gotten better. Though he still said the Pirates were playing beyond their expected win-loss record thanks to some historically great relief pitching, he concluded that "there is no objective reason to expect them to play worse than .500 over the rest of the year."

"This Pirates team probably isn't going to keep the Cardinals at bay, and they might even get passed by the Reds, who are a better team on paper as well," he wrote. "But, they're a solid team that shouldn't be expected to fall apart, and their roster construction could make them a very tough team to deal with in October."