US President Barack Obama said he was \"deeply humbled\" by a visit Sunday to the Robben Island prison where Nelson Mandela spent some of his darkest days in jail. \"On behalf of our family we\'re deeply humbled to stand where men of such courage faced down injustice and refused to yield,\" Obama wrote in a message in the visitors book co-signed by his wife Michelle. \"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.\"