Which “lost city” was explorer Percy Harrison Fawcett searching for when he disappeared in 1925?

Interesting fact:

Percy Harrison Fawcett was not just an explorer, but a soldier and mystic, who many say was the inspiration for "Indiana Jones." Fawcett had heard stories of an ancient civilization buried deep in the Amazon jungle, full of mysterious monuments, crystals and glowing towers. After some encouraging initial finds, he became obsessed with the mission and decided to set out on a full campaign with his son Jack and Jack’s school friend. Some 400 miles along, Fawcett ordered his Brazilian assistants away and penned a letter to his wife telling her: “You need have no fear of failure.” Nothing more was even heard from Fawcett, Jack or his friend, and dozens of recovery expeditions proved fruitless.