Dementors and About Azkaban

'Dementors are among the foulest creatures that walk this earth. They infest the darkest, filthiest places, they glory in decay and despair, they drain peace, hope and happiness out of the air around them. Even Muggles feel their presence, though they can't see them. Get too near a Dementor and every good feeling, every happy memory, will be sucked out of you. If it can, the Dementor will feed on you long enough to reduce you to something like itself - soulless and evil. You'll be left with nothing but the worst experiences of your life. And the worst that has happened to you, Harry, is enough to make anyone fall off their broom.
'Azkaban must be terrible,' Harry muttered. Lupin nodded grimly.
'The fortress is set on a tiny island, way out to sea, but they don't need walls and water to keep the prisoners in, not when they're all trapped inside their own heads, incapable of a single cheerful thought. Most of them go mad within weeks.'

THE MARAUDER'S MAP - Pg 198