Back in the day, Michigan State was home to the “No Fly Zone.”

From 2010-2015, the Spartans routinely had one of the best defensive backfields in college football.

These days, well, they’re spent by Spartans thinking of those days.

It’s difficult to remember the last time Michigan State allowed 300 passing yards per game, which they’re doing so far this year (301.7 per game, to be exact). The No. 10-ranked Spartans have the No. 14-ranked pass defense in the Big Ten.

Dead last.

Rock bottom.

In the basement.

Undefeated. A Big Ten contender. In the AP top 10 for the first time in more than 5 years … and can’t defend against aerial threats.

Imagine if the Spartans found a way to remedy that woe.

It’ll come down to coaching, that’s the only logical conclusion.

Harlon Barnett, a former star Spartans DB, was the architect of Michigan State’s once-vaunted secondary. During his 11-year stint as secondary coach (2007-17), Barnett coached the likes of Darqueze Dennard, Montae Nicholson, Kurtis Drummond and Trae Waynes.

In 2020, Barnett rejoined his alma mater, assuming the same position, but has yet to reproduce previous results.

Michigan State has talent in its defensive backs group. Safety Angelo Grose averages 7.8 tackles per game, while safety Xavier Henderson averages 8.7 stops. They’re up-and-down as cover men, but they’ve certainly displayed upsides. Chester Kimbrough has had moments at CB, and nickel Darius Snow has shown promise. But as a group, the Spartans’ secondary has failed to impress.

In fact, it’s become the team’s only liability.

Quarterbacks have shredded through MSU.

Western Kentucky QB Bailey Zappe threw for an astounding 488 yards and 3 TDs during a 48-31 loss.

Though he didn’t put up similar numbers, Rutgers QB Noah Vedral also found weaknesses in MSU’s defensive backfield. Vedral converted a few 3rd-and-longs, most notably a 19-yard connection to WR Aaron Cruickshank on a 3rd-and-18 in the first quarter.

Adrian Martinez didn’t throw a touchdown, and he only threw for a modest 248 yards — but he did enough to keep Nebraska within striking distance during a close 23-20 loss in Lincoln. Martinez made important throws when necessary, easily taking advantage of gaps in coverage.

Miami’s D’Eriq King threw for 388 yards and 2 TDs during a 38-17 loss to the Spartans. He had 2 picks, but he still flung the ball around with little to no regard for Michigan State’s defensive backs.

Northwestern’s Hunter Johnson threw for 283 yards and 3 TDs during a 38-21 loss.

Nearly every QB has had some type of success against Michigan State, even in losing efforts.

Things will have to change very soon, though. It’s about to get real for MSU’s DBs.

This Saturday, the Spartans could face Indiana QB Michael Penix Jr. (he’s a game-time call), one the Big Ten’s leading passers in 2020. He’s thrown 7 picks this year but still should be considered dangerous — especially with WR Ty Fryfogle at his disposal.

Later this season, they’ll face Ohio State QB CJ Stroud, the Big Ten’s leading passer — and that will be a test, considering Stroud has WR Chris Olave and Company as targets.

They’ll face Penn State’s Jahan Dotson and Sean Clifford, the Big Ten’s current No. 5-ranked passer.

Maryland’s Tua Tagovailoa, the league’s No. 2 QB in terms of yards per game, also will get a shot at Michigan State — which will be in the midst of running through a minefield by playing 3 teams currently ranked in the top 10 of the Associated Press poll in a 5-week period.

Even when the Spartans weren’t in full “No Fly Zone,” they still maintained a position of “bend, but don’t break.”

Through 6 weeks, the Spartans’ secondary has broken far too often. MSU is the only top-10 team from the Big Ten to give up 400- and 300-yard games through the air. Based on the quality of upcoming opposing QBs, it looks like more struggles could be around the corner if MSU doesn’t figure out a way to buck recent trends.

The rest of the season could come down to a few big plays. MSU has had plenty go in its favor. But it’s in danger of being on the other end if it doesn’t sharpen its pass defense.

Michigan State has a flashy offense, but it can’t forget what worked in the past: Defense.

If the Spartans want to truly be taken seriously, they’ll have to get back to the days of old. Barnett might want to watch some old film and reminisce. They have to do something, or their season will be grounded because they couldn’t defend against aerial attacks.