Sandman is a character from Marvel Comics. He was born William Baker, but later adopted the name Flint Marko, and was a criminal that gained dominion over sand after being exposed to radiation. He was part of Spider-Man's rogues gallery, but he eventually became an antihero.
Flint Marko was a criminal, when one day he escaped from prison, but unfortunately was caught up in an atomic explosion. The explosion had mutated Flint into sand particles and was enable to control them. Later, Nick Fury captured him and left him on an isolated island at the Bermuda Triangle. He became lonely at the island and created a playmate for himself, which he called "Sandy".
Flint was visited one day by Spider-Man and the other trainees, who were on vacation. He decided to play with them, but the trainees escaped with the plane from his dangerous actions. Unnoticed by Spider-Man and the others, Sandman hid inside Spider-Man's unitard. He was noticed by them and they landed the plane on the river that was encased in ice. Sandman tried to get on land to destroy Manhattan, but was stopped by Nova, who heated his silica in the sand to glass.
S.H.I.E.L.D. put him in a special hourglass prison that stopped his reformation with vibrations.