The earliest written record of a sumo bout dates from 712 and relates a legend of a contest between two gods, Takemikazuchi and Takeminakata, for control of the Japanese islands. The victorious Takemikazuchi went on to father the imperial family, from where the present emperor traces his roots.
Wall paintings from earlier periods suggest the sport's origins are far older and that it may have began as a ritual to pray for a good harvest.
There are about 70 professional wrestlers at the highest levels of the Japanese national sport, competing in six basho, or tournaments around the country each year.
Boys who show an aptitude for the sport generally join a stable in their mid-teens. But their numbers have been falling in recent years as youngsters prefer to play baseball or football over the regimented life of a sumo wrestler. A match-fixing scandal in the mid-90s further set back the sport.
All unmarried apprentices and wrestlers must live in their stable under the supervision of the stable master and his wife. Privileges come with rank. Juniors have to share rooms, wake up earlier for training and take care of the household chores. They also eat the traditional meal of chanko-nabe stew only after the more senior wrestlers.
Even so, sumo is increasingly popular among foreigners and women at the amateur level, with federations set up in more than 65 countries.