The tropical island of Saipan is experiencing a baby boom.

Saipan is part of the Northern Mariana Islands, a tiny U.S. territory just four hours away from China by plane. It's popular with tourists, because no visa is required for visits up to 45 days.

But an under-the-radar industry is attracting a different kind of tourist.

It starts back in China - where in central Beijing, with one of our producers posing as newly pregnant, ABC News found a tourism agency that sells packages for expecting mothers looking to have an American-born baby. Babies born in Saipan are legally U.S. citizens.

The tour agent we met with sold Saipan like a 5-star vacation, with brochures touting high-end accommodations and top-of-the-line medical care. Full packages started at $27,000, promising social security cards, birth certificates, and most importantly, an American passport with instant citizenship for the baby.

ABC News didn't buy the package but heard that it is easy enough to make the trip without one. So we flew to Saipan ourselves.

Within hours of arriving in Saipan, brokers online were vying for business. Soon they met with our producer - still undercover as a newly pregnant prospective client - to show her places she could stay.

"I know everything you need to know about having an American baby," one broker said during one tour. Two of his clients, two pregnant Chinese women, were also present.

Another broker took our producer on another tour, selling ocean views and offering cooking and laundry while the clients who were already living there were getting ready with baby bathtubs and strollers.

Local records show the number of babies born to Chinese mothers has skyrocketed, increasing 35 fold from just eight in 2009 to 282 in 2012, according to the Health and Vital Statistics office on the Northern Mariana Islands.

That means 71% of babies born in Saipan are "ABCs" - American-born Chinese.

It is not illegal, but there are critics.

"When they come of age, they can apply to bring their families into the United States," said Rep. Steve King of Iowa. "It looks to me that we've got a rather open door in the Northern Marianas and we should figure out how to close it."

Saipan's U.S. congressional representative said that he is aware of the issue and has even written and asked the U.S. Department of Homeland Security for help.

"We want to make sure this small problem stays small," said Rep. Gregorio Kilili Camacho Sablan. "We are trying to fix this. We are trying to control the problem."

They are trying to control it before the business of "ABCs" gets in the way of the larger Chinese tourism industry and the dollars it brings into Saipan's economy.

"Their contribution is truly important," Sablan told ABC News of the tourism money.

Back in China, business is booming, with agencies selling packages for Chinese babies who will always be American.

ABC News' Karson Yiu and Maggy Patrick contributed to this report.

ABC News