The Department of Homeland Security has discovered 67 cross-border tunnels, detected 534 "ultralight aircraft incursions," and monitored 309 drug smuggling incidents involving small fishing boats from 2011 to 2016, according to a new report from the Government Accountability Office.

Nearly all of the cross-border tunnels were discovered in the border regions from Arizona to California, and GAO said 80 percent of the tunnels were "sophisticated and interconnecting."

Like the tunnels, nearly all of the ultralight "aircraft incursions" also were detected along the southwest border with Mexico.

While all of the data point to more sophisticated smuggling methods, DHS and GAO both said that the number of tunnels and aircraft incursions declined gradually over the six-year period.

For two of the DHS agencies, Customs and Border Patrol and Immigration and Customs Enforcement, countering these smuggling threats is partially a technological battle that sometimes takes years. The agencies have worked to acquire tunnel detection technologies as well as radar that can detect ultralight aircraft, but the report from the GAO showed that those technologies still aren't fully mature and also aren't fully ramped up.

Illegal border crossing incidents have declined sharply under President Trump, which he attributes to his tough talk and his effort to start building a wall on the border. Before the election last year, illegal crossings and apprehensions of illegal immigrants soared, as some were hoping to enter the U.S. to benefit from a hoped-for amnesty from Hillary Clinton, or to get in before Trump took steps to seal off the border.