This is part of the America's Future series airing on FOX News Channel, looking at the challenges facing the country in the 21st century.

NEW YORK — There are over 300 official ports of entry in the United States, giant pores where billions of dollars worth of goods flow through every day — giant targets for terrorists seeking to smuggle in weapons for the next attack.

U.S. Customs and Border Protection is the first line of defense, scouring for so-called "dirty bombs" and other weapons of mass destruction.

"Our strategy begins with stationing our officers overseas, and we have over 200 Customs and Border Protection officers around the world in 58 of the largest seaports," said Jay Ahern, deputy commissioner of U.S. Customs and Border Protection.

Click here to see more on confronting the terrorist threat at U.S. ports.

The idea is to "push the borders out" — to have more control of imports and more time to manage potential terror threats before they head to U.S. shores.

All incoming cargo ships are checked electronically, and all crews and cargo are accounted for and logged.

In addition, all vessels coming from foreign ports are boarded by customs agents to make sure that everything that was screened prior to arrival still checks out.

There has never been a terrorist attack on a U.S. port, and since the Sept. 11 attacks, billions of dollars have been spent on homeland security, but some critics say more needs to be done.

"It's a huge challenge to figure out how we going to manage this risk," said Stephen Flynn, a senior fellow at the Council on Foreign Relations and an advisor on maritime affairs for the Bush administration. "And given the scale of the vulnerability and the consequences of a terrorist event, particularly targeted at our ports or cargo coming to our ports, we are just not there yet."

Earlier this summer, Sens. Frank Lautenberg and Robert Menendez (D-N.J.) introduced legislation that would require cargo be monitored from the moment it's packed into containers abroad. Some officials say that's impractical.

"You know there is a certain balance between commerce and being productive in terminals like this and being so safe, you could safe yourself out of business," said Don Hamm, president of Port Newark Container Terminal.

Some lawmakers cautioned that there is no magic formula to guarantee safety at U.S. ports.

"To me there is no silver bullet," said Rep. Peter King (R-N.Y.), ranking member of the House Homeland Security Committee. "Anyone who says you are going to get 100 percent security by doing X, Y and Z is wrong."

Federal, state and local authorities say they are managing the threats to U.S. ports with multi-layered defense tactics. The U.S. Coast Guard patrols and checks vessels coming in with radiation detection devices, as does the NYPD.

In New York, police officials are using new counter-terrorism vessels called "TRACS" boats that are hard-wired with state-of-the-art radiation detection systems, and U.S. Customs scans 98 percent of all cargo entering U.S. ports for radiation.

Customs officials say they continue to work with top scientists and technology agencies to develop cutting-edge technology to stay ahead of terrorists. And even as lawmakers say progress has been made, they continue to press for further advances.

"We can never be happy," said King. "We always have to be going forward because the enemy is constantly adapting. And we know as of September 11th there is a long way to go, but we have gone a long way."

Click here to see more reports on America's Future.