In 2012, Io Shirai and Mayu Iwatani formed the legendary team, Thunder Rock, to enter the Goddesses of Stardom Tag League. After a strong opening match, they lost in the next two rounds, costing them a spot in the finals.
In January of 2013, Shirai, Iwatani, and Natsumi Showzuki competed in a tournament for the inaugural Artist of Stardom Championship and unsuccessfully challenged for the Goddess of Stardom Championship. Although Shirai eventually had a breakout year as a singles competitor, Thunder Rock didn’t turn their fortunes around until 2015 as members of Heisei-gun.
On May 6, they defeated Oedo Tai and Candy Crush in a tournament for the vacant Goddess of Stardom Championship at Golden Week Stars 2015. This reign would become their claim to fame. They held the titles for 407 days with a record-setting 11 title defenses, which shattered the former record of three.
In November, Thunder Rock entered and won the 2015 Goddesses of Stardom Tournament. Shirai and Iwatani subsequently notched their sixth successful title defense in the finals when they defeated Hiroyo Matsumoto and Santana Garrett.
In June of 2016, they lost the titles to Oedo Tai, which set the table for one of the most infamous heel turns of all time. Thunder Rock made it to the finals of the Goddesses of Stardom Tag League, but they lost to Kairi Hojo and Yoko Bito as Shirai uncharacteristically watched apathetically from the apron.
Afterward, Io turned on her longtime tag partner, ending Thunder Rock for good and inciting their rivalry. Of course, they both flourished as singles competitors but their dominant run as a tag team will always be a career highlight for both Io Shirai and Mayu Iwatani.