Chosen by commenter Alpha75, Matthew David Pendergrast disappeared from Memphis, Tennessee on December 1, 2000. He was 23 years old and within weeks of graduating from Rhodes College when he dropped off the map.
From writings Matthew left behind when he disappeared, it looks like he might have had a nervous breakdown of some kind. Schizophrenia and bipolar disorder both usually develop in a person’s late teens or early twenties. However, the cops don’t seem to be buying into the mental illness theory and are focusing on possible foul play instead.
Disturbingly, Matthew’s journal said something about going into the water, and they found his vehicle and the clothes he’d been wearing near a swamp. Whatever happened to him, he was probably naked or nearly so.
I did find this long feature article about Matthew’s disappearance in Memphis Magazine.