The NFL's International Series shifts to Mexico City this week, and coverage will focus on the enormous support from local fans. Estadio Azteca will be sold out on Monday night as the Houston Texans and Oakland Raiders face off, a confirmation of the continued interest in American football outside United States borders. Less visible, but no less interesting, is the rise in player participation around the globe. A new adult league debuted in Mexico City this spring, joining dozens of semi-professional and club-level leagues in every corner of the planet -- from Europe to Asia to Australia.
The worldwide action is tracked daily by American Football International (AFI), a website and advocacy group run by Roger Kelly and John McKeon. Kelly is a former Canadian Football League executive, now living in Sweden. McKeon, a former player in a French league, is based in New York.
ESPN.com teamed with AFI to assess the quality and scope of Planet Football. What follows is a subjective ranking of the top countries from a standpoint of participation and local interest.
Key stat: Canadians play with 12 rather than 11 players on each side, helping to fill up a field that is both longer (110 yards) and wider (65 yards) than traditional American football.
Fun fact: The first documented "American" football game in Canada took place in 1861, a full eight years before the first documented "American" football game in the United States.
Who is playing: The nine-team CFL offers a unique spin on the game, and it plays at a high enough level that a handful of players move on to the NFL after each season. Each team is allowed 19 international -- i.e. American -- players. The rest come from Canada's extensive CIS college system, which features 27 schools from four regions that play at its highest level. The CFL once felt comfortable enough to expand into the United States, an experiment that ended in 1995 but led to a firmer vision for a regionally focused league.
Key stat: There are 54,000 federation members among 450 club teams.
Fun fact: Former Duke and Fresno State quarterback Brandon Connette was Germany's top quarterback in 2016. He threw for 4,747 yards in 17 games for the Dresden Monarchs.
Who is playing: Germany proved to be the most fertile ground for NFL Europe, and eventually five of the league's six teams migrated within its borders. That popularity carried on even after the 2007 shuttering, and the best club teams in Germany now play at a level equivalent to a Division II college team in the United States. This spring, Germany produced its first NFL draft pick: wide receiver Moritz Bohringer, who has spent the season on the Minnesota Vikings' practice squad. Bohringer was the first international player to move directly from a European league to the NFL.
3. Great Britain
Key stat: British American Football Federation has nearly 12,000 members.
Fun fact: There is a five-on-five women's tackle league in Great Britain that has a tradition known as the Sapphire Series, which is now taking applicants for the 2017 tournament.
Who is playing: NFL interest has soared in London and the surrounding areas since the start of the International Series in 2007. This year, a total of 242,373 fans attended three games held at two different stadiums. College football has grown dramatically throughout the United Kingdom, an important development. The UK's National League is fully amateur, meaning no paid American imports are allowed. So for the league to thrive at a post-college level, it needs native talent to surface. University teams are poised to provide that talent pool.
Key stat: The first organized American football game in Mexico took place in 1896.
Fun fact: America's Team was once Mexico's Team as well. Perhaps that's still the case. The Dallas Cowboys, also known as Vaqueros de Dallas, began appearing on Mexican television in the 1960s. Hispanic place-kicker Danny Villanueva, a New Mexico native, was a big draw.
Who is playing: American Football has a long history in Mexico. The first games were played there at the collegiate level in the 1920s, and Mexico is now one of two countries outside of the United States to support a fully professional league. (Canada is the other.) The Liga Futbol Americano debuted in the spring of 2016 with four teams and recently announced expansion into a six-team league for 2017. Most of the league is based in Mexico City. Geographical proximity to the NFL makes it a more natural transition for fans. A then-record 103,467 filled Estadio Azteca for the 2005 game between the Arizona Cardinals and San Francisco 49ers. Two preseason games there in the 1990s drew a combined 218,800 fans, and this season's game -- with a reduced capacity of around 87,000 following renovations -- sold out within minutes.
Key stat: Approximately 4,000 people in Austria are licensed to play either flag or tackle football.
Fun fact: The Vienna Vikings played in five consecutive Eurobowls, winning four of them, from 2003-07. That's four more championships than their American counterparts, another Vikings franchise with whom they share an almost identical logo.
Who is playing: The Austrian Football League rivals Germany for the highest-level football played outside North America. AFI currently ranks the Swarco Raiders as the top team in Europe. The Raiders' origin goes back to the 1980s, dating to the earliest days of NFL's excursion into Europe. For a long stretch, Austrian teams dominated international competitions. During the Vikings' run in the mid-2000s, their championship games were televised. Their games were played in front of crowds that were as large as 10,000 fans, and the entire league continues to draw well.
Key stat: There are 18 teams in Japan's X-League.
Fun fact: American football was first introduced to Japan in 1934 by the legendary missionary Paul Rusch.
Who is playing: The highest levels of football in Japan are organized through corporations that sponsor the teams and "staff" them with their own employees. Companies that field teams include IBM, Fujitsu, Panasonic and Mitsubishi. A handful have developed into semi-pro-level teams, and the Japan Bowl will be played in December in the Tokyo Dome. Japanese teams have grown increasingly aggressive in recruiting American-born players. The IBM BigBlue roster, for example, includes quarterback Kevin Craft (San Diego State/UCLA) and receiver/tight end John Stanton (St. John's of Minnesota).
Key stat: Brazilians have played tackle football in an organized way for only eight years, but there are already 105 teams and approximately 8,500 members of its federation.
Fun fact: Team names in Brazil are fantastic, as long as you are not prone to phobias. There are clubs known as the Rednecks, Tsunami, Crocodiles, Vipers, Black Sharks and Brown Spiders.
Who is playing: Although the level of play is not high relative to other countries, fan support is wild for teams in the CBFA Superliga -- a structure devised this year to pit the top 31 teams in the country for a regular rotation. As many as 15,000 fans show up for games, rivaling soccer events in some cases. The Corinthian Steamrollers have 1.4 million fans on their public Facebook page. As a result, Brazil is viewed as perhaps the most fertile ground for grassroots expansion in the world.
Key stat: The game was introduced about five years ago on a significant scale, and as of last summer, about 5,000 men and women were playing in some form. That includes the inaugural Chinese Arena Football League, backed in part by the owners of the Philadelphia Soul.
Fun fact: Former Arizona State defensive lineman Zach Brown, living in China for work, started a semi-pro league by handing out business cards to people he met on the street. The cards said, "I play American football. You look like a dude who could play. Contact us here." Runner-up: The Shanghai "Skywalkers" are a real thing. Enough said.
Who is playing: Ken Li, the founder of the American Football Union, said China is more interested in the NFL than playing the game itself. The level of play here "is no comparison to the NFL at all," Li said. He added: "I always say that if we put the best players in China on an all-star team, the level might be similar to a Division III in the NCAA. That gives you the best-case scenario of what we're seeing here."
Key stat: France has a huge if moderately competitive federation, counting 215 registered clubs and 22,136 members.
Fun fact: Defensive end Micah Robinson, a former Syracuse star, had 18 sacks last season for Saint-Ouen-l'Aumône Cougars, the top team in France based on AFI's rankings.
Who is playing: Soccer and rugby dominate the French sports terrain, so support is mixed even while participation and skill level is high. Most football teams have American imports playing quarterback, including many with experience at Division I schools. There have also been occasional instances of French-born players piquing interest of NFL teams. Receiver Anthony Dable, who like Bohringer discovered football on YouTube and went on to play in the French and German leagues, spent training camp with the New York Giants and had a September tryout with the New England Patriots.
Key stat: There are approximately 4,000 members in the Australian federation.
Fun fact: The first season of American football in Australia was 1984. Players used boxing head gear instead of helmets and played on what Gridiron Australia's official history called a "disused rubbish tip." In other words, it was an abandoned garbage dump.
Who is playing: American football has plenty of team-sport competition in Australia, from rugby to Australian rules football to soccer. But there is a growing list of Australian-born players working their way into the NFL. Former rugby star Jarryd Hayne is the most notable, having played last season as a running back/returner for the San Francisco 49ers. Most of them have been specialists, however, including current New York Giants punter Brad Wing. Sav Rocca had a seven-year career with three teams, and Mat McBriar punted for 11 years, mostly with the Dallas Cowboys. And Darren Bennett, the San Diego Chargers' punter from 1995-03, introduced rugby-style kicking to the NFL.