On March 2, 1995, Fermilab announced the discovery of the top quark, the last of the six predicted quarks. The search began in 1977 when physicists found the fifth quark, bottom, at Fermilab. It took this long because the top quark was much more massive than was originally imagined, so it required a more powerful accelerator to create it.
Although the top quark decays too fast to be observed, it does leave behind particles that give evidence of its existence - a top quark "signature". The top quark can decay in more than one way. Since a top quark appears only once in several billion collisions, it was necessary to perform trillions of collisions.
Physicists still do not understand why the top is so massive. It is 40 times heavier than the next heaviest quark and about 35,000 times heavier than the up and down quarks that make up most of the matter we see around us. In fact the question still remains why particles have such different masses.