Now Playing: Solving the North Korea problem

Now Playing: President Trump says North Korea has 'gotta behave'

Now Playing: Inside North Korea, the world's most reclusive country

Now Playing: Eiffel Tower goes dark in memory of Barcelona terror attack victims

Now Playing: SPECIAL REPORT: 13 dead, over 50 injured in Barcelona terror attack

Now Playing: Clouds cause Chinese forest park to look eerie

Now Playing: Eyewitness describes chaotic scene after Barcelona attack

Now Playing: SPECIAL REPORT: A van drove through a crowd in Las Ramblas in Barcelona, Spain

Now Playing: Rex Tillerson offers condolences after terrorist attack in Barcelona

Now Playing: SPECIAL REPORT: At least 1 dead in Barcelona terror attack

Now Playing: Prince William opens up about Princess Diana's funeral

Now Playing: Police search streets of Barcelona after truck attack

Now Playing: Hostage situation unfolding in Barcelona after vehicle hit pedestrians

Now Playing: Disturbing scene after truck hits crowd in Barcelona

Now Playing: US general: North Korean nuclear attack is 'unimaginable'

Now Playing: SPECIAL REPORT: Truck hits pedestrians on busy Barcelona street

Now Playing: Police respond to truck hitting pedestrians on busy Barcelona street

Now Playing: Truck hits pedestrians on busy Barcelona street

Now Playing: 'Hora Cero': Venezuela's crisis through a mother's eyes

Now Playing: Lost engagement ring turns up in family's garden after 13 years