(Credit: JIM WATSON/AFP/Getty Images)

CAPE TOWN, South Africa - With the world's eyes on the ailing Nelson Mandela, President Obama today walked in the footsteps of the man he's called his personal hero.

Touring the prison on Robben Island, Obama stood alone in the stark "7B" cell where Mandela was imprisoned for 18 of his 27 years in captivity and stared out at the blue sky through the barred window.

Obama visited the island before, in 2006 when he was a senator, but today he returned for the first time as president and brought along his family.

"For me to be able to bring my daughters there and teach them the history of that place and this country, and help them to understand not only how those lessons apply to their own lives but also to their responsibilities in the future as citizens of the world, that's a great privilege and a great honor," Obama said Saturday.

The first family viewed the quarry where Mandela and his fellow prisoners were forced to do grueling labor, endlessly breaking large stones into smaller ones, and the courtyard provided for their recreation. They walked through the tiny, barren cells, which were barely wide enough for the prisoners to lie down.

The president, first lady, daughters Sasha and Malia, first grandmother Marian Robinson and the president's niece Leslie Robinson were guided through the prison by a former inmate. Like Mandela, Ahmed Kathrada was imprisoned for 18 years for his anti-apartheid activities.

The president was overheard chiming in with his own history lessons.

"One thing you guys might not be aware of is that the idea of political nonviolence first took root here in South Africa because Mahatma Gandhi was a lawyer here in South Africa. Here is where he did his first political [activism]. When he went back to India the principles ultimately led to Indian independence, and what Gandhi did inspired Martin Luther King," he told his family.

Before departing, the president and first lady signed the visitors log.

"On behalf of our family we're deeply humbled to stand where men of such courage faced down injustice and refused to yield. The world is grateful for the heroes of Robben Island, who remind us that no shackles or cells can match the strength of the human spirit," the president wrote.