The Eden Project’s huge domes feature plant life from around the world to highlight humanity’s reliance on nature.
As they were shown around the site, Charles, a former Duke of Cornwall, said: “I remember when I first came here there was nothing – amazing how it’s developed and grown.”
There had been laughter earlier when the King joked about his arrival – his state limousine had made a wrong turn on the sprawling site and disappeared momentarily.
Getting out, Charles quipped: “Now you see us, now you don’t.”
During the visit, the couple launched the Eden Project’s The Big Lunch and The Big Help Out – street party-style events to be held from 5 to 8 June.
As they met volunteers and community groups for lunch, they were tasked with cutting a cake depicting a street party scene.
Colonel Sir Edward Bolitho, Lord Lieutenant of Cornwall, handed his sword to the couple – who perhaps found it a little unwieldy for the job.