We're nearing the end of our Big Ten position rankings, and it's time to finish up the defense rundowns with a look at the secondaries. Let's start off with the unit rankings.

As a reminder, we're basing these mostly on last year's performance and who returns, along with potential for the 2012 season.

The top four groups could be very good, while the next five have question marks but potential. Even the bottom three groups have realistic opportunities to make strides this fall.

Let's get rolling ...

1. Michigan State: The Big Ten's most formidable defense once again should be very strong in the back four. Although All-Big Ten safety Trenton Robinson departs, Michigan State returns its other three starters, led by standout cornerback Johnny Adams. Some project Adams as a potential first-round pick in the 2013 NFL draft. Safety Isaiah Lewis could have a breakout season, and the Spartans have recruited well here to build good depth.

2. Ohio State: The defensive line has bigger names and more hype, but the secondary might turn out to be Ohio State's best unit in 2012. The Buckeyes bring back all four starters, including arguably the league's top cornerback tandem in Bradley Roby and Travis Howard. Expect Roby to take another big step as a sophomore. Hard-hitting safeties C.J. Barnett and Christian Bryant return, and Ohio State can go two- or three-deep at most positions.

3. Michigan: This group has come a very long way from the Rich Rodriguez era and should be the strength of Michigan's defense in 2012. Safety Jordan Kovacs is an excellent leader who blossomed in Greg Mattison's system last fall. The Wolverines also boast a promising cornerback tandem in J.T. Floyd and Blake Countess, and have good overall depth at both corner and safety.

4. Nebraska: While the Huskers lose the Big Ten's top defensive back in Alfonzo Dennard, they should have greater overall depth and the potential for new stars to emerge. Hard-hitting safety Daimion Stafford leads the group, and P.J. Smith provides a veteran presence at the other safety spot. Nebraska is loaded with options at cornerback, including the improved Andrew Green and juco arrival Mohamed Seisay. New assistant Terry Joseph should get a lot out of this group.

5. Purdue: The rankings already have mentioned some good cornerback tandems, and Purdue adds another in Ricardo Allen and Josh Johnson. They've combined for 48 career starts, and Allen has led the team with three interceptions in each of the past two seasons. Max Charlot returns at safety after recording 41 tackles in 2011, but there are some question marks around him.

6. Illinois: Terry Hawthorne rarely gets mentioned as one of the Big Ten's top defensive backs, but he should. The senior has been a natural playmaker throughout his career and will lead Illinois' secondary in 2012. Senior Justin Green brings experience to the other corner spot. Although the Illini return both of their starting safeties -- Steve Hull and Supo Sanni -- they need more consistency from that position this fall.

7. Wisconsin: The Badgers lose a key player at both cornerback (Antonio Fenelus) and safety (Aaron Henry), but they have a chance to improve upon last year's performance and rise up these rankings. They'll undoubtedly benefit from the return of cornerback Devin Smith from injury. Head coach Bret Bielema doesn't downplay what Smith's absence meant last season. The Badgers need more consistency out of projected starters Dezmen Southward and Marcus Cromartie.

8. Iowa: The Hawkeyes have a nice piece to build around in playmaking senior cornerback Micah Hyde, but they'll need more after a so-so season in 2011. Tanner Miller returns as a starter at safety, and hopes are high for junior B.J. Lowery at the other corner spot. Iowa's depth looks better at corner than it does at safety.

9. Penn State: Most see the secondary as Penn State's weak link, to which Malcolm Willis and Stephon Morris say, "Bring it on." Still, the Lions have questions to address after losing all four starters from the 2011 team. Morris, Willis and sophomore Adrian Amos all have been in the fire a bit, but Penn State needs them to take steps and remain on the field. Depth is a significant concern after the offseason departures of Curtis Drake and Derrick Thomas.

10. Minnesota: This is a bit of a projection pick, but I like Minnesota's potential to take a step forward in the secondary this fall. The biggest reason for optimism is cornerback Troy Stoudermire, who returns for a fifth year after missing most of last season with a foot injury. Stoudermire was on track for a big year before the injury. Cornerback Michael Carter had a strong spring and could finally reach his potential. The bigger concerns here come at the safety spots.

11. Northwestern: Three starters depart from a secondary that struggled to stop anyone and endured major communication breakdowns far too often in 2011. Northwestern is younger in the back four, but it also could be more talented this season. Sophomore safety Ibraheim Campbell comes off of a 100-tackle season, and cornerback Nick VanHoose impressed during the spring. A few veterans return, but the coaches can't be afraid to go with the youth movement here.

12. Indiana: The Hoosiers finished eighth in the Big Ten in pass defense last fall, but only because teams had their way with IU on the ground. Indiana surrendered a league-high 26 pass touchdowns and only recorded five interceptions. There's hope, though, as the Hoosiers return three starters, including top cover man Lawrence Barnett. If Mark Murphy and Greg Heban make strides, and some newcomers help right away, Indiana could be decent in the back four.