Typically, border patrolling is thought of as an effort to keep people on the proper side of the border. However, it also entails illegal objects, such as dinosaur fossils stolen from Mongolia over the years. A black market has formed around the smuggling and sale of dinosaur skeletons, leading to the disappearance of very rare skeletons and fossils.
Back in 2014, US officials first recovered parts of dinosaur skeletons that were sold at auction by Eric Prokopi, a paleontologist from Virginia, who pled guilty to charges and aided authorities by tipping them off to other fossils and skeletons that were floating around the black market.
In another instance involving Nicolas Cage, the actor purchased a Tyrannosaurus skull back in 2007 for a private collection. Authorities discovered later that the skull may have been stolen. Cage was not blamed for purchasing the skull, but he was asked to return it. He did so in late 2015. Cage won the skull by reportedly outbidding actor Leonardo DiCaprio in 2007.
One of the other fossils recovered was an Aliomarus skull, of which there are only three in the whole world. It was caught at the US border after being sold on eBay by a French dealer who marketed it as a French replica.
Just recently, in April 2016, U.S. Immigration and Customs Enforcement held a ceremony commemorating the collection and return of about 30 fossils to Mongolia. Many of the skeleton remains are a part of Mongolian history and are too rare to be ethically owned by a single person, so they have been placed in Mongolian museums for public viewing.