Former Pittsburgh Steelers defensive tackle Joe Greene is picky when it comes to watching NFL games. When he plops down on the sofa in his suburban Dallas home, the Sunday action usually doesn't hold his attention for long. He'll fidget with his remote, scroll through the channels and then roll his eyes at the mediocrity displayed on his big screen. The only time Greene, a Hall of Famer who anchored one of the best defenses in history, is consistently entertained is when the Seattle Seahawks are playing.

That's when he'll lean forward in his seat and focus on the activity. When Greene watches Seattle's defensive players flying around the football, he has the expertise to accurately assess what he sees: a unit that might go down as one of the best ever.

"You really have to search to find a good game when you watch the NFL these days," Greene said recently. "You usually get one when you're watching that team play."

The Seahawks have taken their defense to such rarefied heights that Greene can't help but respect their success. This is the third consecutive year Seattle has led the league in scoring defense, something that hasn't happened in the NFL since the Minnesota Vikings did it from 1969 to 1971. Seattle also has been so dominant of late -- it allowed an average of 6.5 points over its last six regular-season games and now prepares to meet Green Bay in the NFC Championship Game -- that Greene could have a hard time finding a defense worth admiring in the foreseeable future.

More than ever, the NFL's balance of power is tilted toward offense. And the further the league pushes its rules and officiating toward higher scores and increased safety, the more it feels like the Seahawks could be the game's last great defense.

"You never want to say never," said former Chicago Bears middle linebacker Mike Singletary, another Hall of Famer. "But I will say this: It's more difficult than ever to play defense today, and I think it's probably easier than ever to play quarterback."

Said ESPN analyst Phil Savage, who has worked as a personnel executive with the Baltimore Ravens and as the Cleveland Browns' general manager: "The deck is really stacked toward the offense in today's game. You see that when you look at the talent, the rules and, in some ways, the coaches. The coaches who can score points have the higher profile. I just don't know if it will be possible to accumulate enough players to have great defenses anymore. In some ways, it's easier to build a dominant offense than a dominant defense."

Ever since the NFL opened up the passing game in 1978 with rule changes -- including prohibiting defenders from making contact with receivers more than 5 yards beyond the line of scrimmage -- there have been only a handful of great championship defenses. The Steelers still had enough core members of their vaunted "Steel Curtain" to add two more Super Bowl championships to the ones they captured in the 1974 and '75 seasons. Singletary was the leader of a Bears defense that destroyed opponents from 1984-86 and led that team to a championship in 1985. The 2000 Ravens, the 2002 Tampa Bay Buccaneers and the 2008 Steelers also had stellar defenses that powered Super Bowl runs.

None of those teams had to deal with the way offenses are treated in the NFL today. Rules make it nearly impossible to punish quarterbacks and illegal to pummel defenseless receivers. There are more freakish talents lining up at wide receiver and tight end than ever before, with teams far more willing to play four- and five-receiver sets. Heading into this season, there had been 15 available head-coaching jobs over the past two years, and only four went to men who made their names coaching defense: Jacksonville's Gus Bradley, Tampa Bay's Lovie Smith, Minnesota's Mike Zimmer and Cleveland's Mike Pettine.

All-Pro cornerback Richard Sherman is one of the late-round picks who has thrived under Pete Carroll. AP Photo/Scott Eklund

The Seahawks have thrived in this environment largely because they hired coach Pete Carroll in 2010 and gave him power to run the team as he saw fit. Stories about his first year in Seattle are now legendary, as he and general manager John Schneider overhauled the roster with roughly 200 transactions. Carroll wasn't swayed by the push toward more prolific offenses -- "People have changed around the league because a lot of people like to throw it, but [being physical] still works for us," he said during a 2013 interview -- and he focused on finding hidden gems to fulfill his vision. The Seahawks' vaunted secondary was built with only one starter, free safety Earl Thomas, drafted higher than the fifth round.

The Seahawks drafted so well the first five years of Carroll's tenure that they've put themselves in position to meet the first major criterion for sustaining greatness -- a wealth of young talent. Defensive tackle Kevin Williams is the only key contributor over age 30 (he's 34), and Seattle already has Pro Bowlers Thomas, cornerback Richard Sherman and strong safety Kam Chancellor under contract through at least 2017.

"That's what you have to do if you want to be great," said Hall of Fame linebacker Derrick Brooks, who played on the Buccaneers' elite defenses in the late 1990s and early 2000s. "We had to attain and sustain when we were in Tampa, and Seattle is doing the same thing. We drafted young guys, paid them early and wound up with about seven to eight core players for our defense."

SEATTLE'S PLACE IN HISTORY How the Seahawks defense compares to selected NFL-best scoring defenses in relation to the No. 2 scoring defense and league average during those seasons: Year/Team PPG No.2 PPG Avg. PPG '14 Seahawks 15.9 17.6 22.6 '13 Seahawks* 14.4 15.1 23.4 '08 Steelers* 13.9 14.6 22.0 '02 Buccaneers* 12.3 15.1 21.7 '00 Ravens* 10.3 11.9 20.7 '85 Bears* 12.4 16.4 21.5 '77 Falcons 9.2^ 10.4 17.1 '76 Steelers 9.9 12.6 19.2 '75 Rams 9.6 11.6 20.6 '71 Vikings 9.9 10.0 19.3 *Won Super Bowl ^NFL record Source: Pro-Football-Reference.com

That might sound easy, but few teams have been able to do it. Also, Seattle has built its defense, at a time when it's becoming harder to find standouts on that side of the ball. Savage recently conducted a study as part of his role as executive director of the Senior Bowl. His research concluded that there has been a major shift in the way high school and college coaches handle talent.

Said Savage: "If you go back to the 1980s and 1990s, you would find a lot of coaches putting their best athletes on defense, which is what happened with a guy like Deion Sanders. But as the game has changed to more three- and four-receiver sets, that talented cornerback can be neutralized to the point that high school coaches are now putting their best athletes back on offense. Those coaches know they have to score points, so they want as much ammunition as possible on that side of the ball.

"If you go to the combine, you'll see that wide receivers are getting bigger and faster every year, while cornerbacks are getting smaller and slower. It's basically getting harder to find elite defensive backs who can play at this level."

That isn't the only major challenge to building a great defense these days. Brooks mentioned the ability to retain coaches had plenty to do with Tampa Bay's success. Former head coach Tony Dungy began building that defense when he arrived in 1996, but defensive coordinator Monte Kiffin remained there for seven more years after Dungy was fired following the 2001 season. The Bucs also had several talented assistants during that period -- including Lovie Smith, Rod Marinelli, Herm Edwards, Mike Tomlin and Gus Bradley -- who later became head coaches. It's rare for any organization to land that many coaches with promising futures.

The Seahawks already have discovered how hard it is to keep a staff intact. Bradley served as Seattle's defensive coordinator from 2009 to 2012 before becoming Jacksonville's head coach. Now his successor, Dan Quinn, is one of the hottest names for teams currently looking for a head coach.

Michael Bennett, with seven sacks this season, and Seattle's D-line don't need a blitz to pressure QBs. Joe Nicholson/USA TODAY Sports

"We were lucky to have Monte for [13] years," Brooks said of Kiffin. "It's not that easy to plug in coordinators and continue playing at a high level."

One would expect more teams to mimic what Carroll has created in Seattle, given how the NFL is known for copycat tactics. The reality is Carroll's brilliance is incredibly hard to replicate, for reasons that go beyond personnel or rules. When Greene talked about the greatness of the Steelers' defenses of the '70s, he didn't just talk about a talent pool that produced four future Hall of Famers. He specifically alluded to tough moments that grew and melded that unit into one for the ages.

In Greene's eyes, the Steelers' defense became the Steel Curtain on a six-hour plane ride from Oakland to Pittsburgh at the end of the 1973 season. The Raiders had beaten the Steelers 33-14 in a divisional playoff game. That defeat would be the final bit of motivation for a defense that already had nine starters in place on a unit that would help win four Super Bowls.

Bruising safety Kam Chancellor, shown celebrating his 90-yard pick-six against Carolina, is under contract through the 2017 season. Otto Greule Jr/Getty Images

"That loss gave us a feeling we never wanted to experience again," Greene said. "That's what happens when you have enough guys who can stay together and go through that. Today, with free agency, you don't have that opportunity as often. You can add two or three great players to your defense, but they won't know what that [pain] feels like."

Seattle had such a moment in the 2012 playoffs, when it lost to Atlanta in the divisional round. Sherman has talked about how critical that defeat was to the Seahawks' maturation, how it helped them focus on claiming the Lombardi trophy last season.

Seattle's confidence also comes from the system it has been running the past five seasons. Carroll and Quinn prefer to stick primarily with a three-deep zone and man coverage, the idea being to pressure quarterbacks without blitzing and to close off passing windows with long defenders on the back end. The beauty of the system is its simplicity. At a time when productive pass-rushers and cornerbacks can sign fat contracts, the Seahawks have bought into the idea of playing what Greene calls "true team defense, which is something you don't see very often these days."

Said Singletary: "One of the biggest challenges to defenses today is finding creative ways to communicate, and Seattle can do that. Teams have so little time to work together in the offseason that they need to learn how to talk to each other on the field.

"Their safety [Thomas] helps them there, but every team has to make a tough decision when it comes to using more volume or less volume in the playbook. They have to ask themselves if they should use every bullet in the gun or whether to go with two or three bullets and focus on certain things every down. A lot of coaches decide to use everything they have."

Singletary and the Bears faced a similar decision when defensive coordinator Buddy Ryan was installing his 46 defense. That scheme was based on the notion of pressuring the quarterback from a variety of angles while playing plenty of man coverage behind it. But in 1984, the Bears were having so much trouble learning the intricacies of the system that players began griping about its complexity. Because Ryan didn't like using players who didn't know their jobs, the Bears were even more frustrated by watching talented members of the roster ride the bench on game days.

Linebacker Bobby Wagner, a second-round pick in 2012, was named to his first All-Pro team this year. AP Photo/Elaine Thompson

It wasn't until several teammates asked Singletary to talk to Ryan about the matter that the coach simplified his defense. A year later, the Bears lost just one game all season with a defense that is often mentioned as the best ever.

"The players kept saying there was too much stuff in our playbook back then," Singletary said. "So I became the guinea pig who had to go tell Buddy what they felt. I kind of told him through a crack in the door. But after he changed some things, that's when we took the next step as a defense."

The Ravens took that step midway through their 2000 championship season. Saddled with an offense that went five consecutive games without scoring a touchdown, the Ravens ramped up their defensive play so much that they ended the year with NFL record lows for points (165) and rushing yards (970) allowed in a 16-game season. Savage sees similarities between the personnel on that Baltimore squad and Seattle's roster -- Bobby Wagner is a rangy, athletic middle linebacker in the mold of Ray Lewis, while Thomas has Rod Woodson's instincts and playmaking ability at safety -- but it's the comparable personalities of the units that jump out at the former Ravens executive.

"It's more difficult than ever to play defense today, and I think it's probably easier than ever to play quarterback."

Specifically, Savage respects the Seahawks' ability to tap into the desire of the players they acquire. "Everything I hear about Dan Quinn is that the players really like him up there, and that's how it was for our players with [former Ravens defensive coordinator] Marvin Lewis," Savage said. "Even though we had some big-name players, we also had Tony Siragusa, who was an undrafted free agent. People said Peter Boulware was too soft, Duane Starks was too little and Rod Woodson was too old. Marvin really tapped into the chips on the shoulders of those players. The Seahawks are doing the same thing by getting guys who are really hungry and motivated."

Those Seattle players haven't had any problems telling the world how good they've been lately. After Seattle beat Arizona on Dec. 21, defensive end Michael Bennett told NFL Network the Seahawks were "the best defense to ever play football."

Added Wagner: "We do recognize what we're doing right now is pretty good. Our job is to keep that going and give you guys lots to talk about. If we put a couple more championships up there, then people should consider us with the great defenses."

Time ultimately will tell where the Seahawks rank in this conversation. Thomas and Sherman are two of the best players in football, but Edwards, now an ESPN analyst, noted, "The great defenses tend to have anywhere from two to four Hall of Famers, and they usually have a dominant player at every level." It's also fair to wonder how long the Seahawks will be able to hold on to their core defensive players, especially given that they have a quarterback, Russell Wilson, who will command a hefty contract in the near future.

"I don't know how long they will be great," one NFC general manager said. "They're already not as good as they were last year because of the players they've lost."

We also have to wonder what will happen when Seattle's run eventually ends. This is an era in which defensive masterminds such as New England's Bill Belichick and former Denver coach John Fox have won mainly with high-powered offenses operated by Hall of Fame quarterbacks instead of stout defenses. The Bears, long known for their defensive history, just fielded one of the worst units in football, and legendary Steelers defensive coordinator Dick LeBeau just resigned after back-to-back seasons with mediocre results. There's little question that offense is going to rule the day for decades to come in the NFL. The real issue is what's going to happen when these Seahawks are no longer around.

There will always be good defenses. But great ones almost certainly will be a lot harder to come by.

"People need to understand that Seattle didn't just find this success suddenly," Greene said. "They were under .500 the first year Carroll was there, and then he found enough players to commit to what he was trying to do. You just don't make something like that happen overnight. It really takes a lot of work to do what they've done."