This season of “America’s Got Talent” has featured a rat that can navigate a New York cityscape, 76 Texas high schoolers who kick as high as an oil derrick and a couple that throws knives at each other while singing opera.

But the most impressive feat for NBC’s amateur competition series has little to do with the performers: It’s that 10 million people regularly watch it.

Broadcast ratings tend to plummet in summer as fewer Americans stay home to watch TV. CBS’s much-hyped “Love Island,” starring beautiful strangers engaged in romantic melodrama, never managed to reach 3 million viewers for any episode this summer.

Yet “Talent,” which begins its popular live-show episodes this week, flies in the face of much of what experts know about the modern television business. Adding to the achievement, much of the audience is in the coveted 18- to 49-year-old demographic and also clicks on YouTube performances in massive numbers.

“I’m not sure what to say,” said Howie Mandel, a longtime “Talent” judge, when asked for theories about the show’s popularity. “I guess viewers just like seeing ordinary people do extraordinary things.”

How the show has become a juggernaut is one of the entertainment world’s biggest puzzles. And no other network is sure how to solve it.

“You’re never going to beat it. But we still try every year,” said an executive at another network who spoke on condition of anonymity.

If you aren’t familiar, “America's Got Talent” is a vintage talent show with “American Idol”-style judging. It involves a host of acts — magic, singing, dancing, comedy — that compete in various rounds, with the goal of advancing to the live-show episodes in August and September. The winner nabs $1 million (albeit paid over 40 years) and performs at a variety show in Las Vegas.

A few have gone to bigger stages, such as magician Shin Lim, last season’s winner, who has since headlined a show in Vegas and appeared on Broadway. Recording stars are rarer.

Backstories are often highlighted and incorporated sentimentally into the performance. This year, one of the acts to make it through to the live show is Voices of Service, an a capella group of military veterans who sing covers of Kleenex-yankers such as Charlie Puth’s “See You Again.”

The show was created by Simon Cowell, who launched a “Got Talent” franchise that would originate on the U.S. airwaves and eventually be exported to dozens of countries. (It is produced by Fremantle Media, the reality juggernaut.) The show began on NBC in 2006, and its first-ever episode nabbed 12 million viewers. Its bona fides were quickly established.

This season, “Talent” has averaged nearly 10 million viewers in its two-hour Tuesday slot. Since it debuted in May, it has been the top show of the night nearly every time it has aired, thanks to performers such as the knife-throwers (Nick & Lindsay), the dance troupe (Emerald Belles) and that rat act (Melissa Arleth and her crafty rodent, Hanta).

This past Tuesday, the show's 9.6 million viewers were nearly triple the number of ABC rival “Bachelor in Paradise” and almost five times that of “Love Island.”

“Talent” also has held the line this year despite the swapping in of several regular judges (Gabrielle Union and Julianne Hough have replaced Mel B and Heidi Klum) and host (Terry Crews for Tyra Banks). Mandel and Cowell remain the stalwarts at the judging table.

One of the keys to the success of “Talent,” executives say, has been its popularity across demographics.

“It’s the rare show that provides a co-viewing experience — my kids, who are 6, 4 and 2, watch it, my husband enjoys it, my parents enjoy it,” said Jenny Groom, executive vice president of alternative programming and development at NBC. “There is a plethora of content nowadays that can divide people. Someone watches on their iPad and someone is watching television and someone else is watching on their laptop. This show brings a whole family together.”

To achieve its unifying effect, producers scour the country for talent. They often spend months on the road to find new acts.

“The goal is to find a new crop every year, to find things people haven’t seen before,” said Sam Donnelly, co-executive producer of the show with Jason Raff.

“When we’re traveling to Chicago or Cleveland, we don’t know what we’ll find,” Raff said. “Many times it’s mediocre or boring,” he added.

“Or weird,” Donnelly said.

“And then occasionally someone comes into an audition room and dazzles us,” Raff said.

The future success depends on just how increasingly audacious Americans are willing to get.

“That is the thing that keeps Sam and I up every single night, to be honest,” Raff said. “It gets harder and harder every year.”