A predawn series of underground explosions ripped through the sewer system beneath the Old Louisville section before dawn yesterday, blasting craters in streets and doing milllions of dollars of damage to homes, businesses and automobiles. Four people were injured, none seriously.

Mike French, director of the city works cabinet, estimated private property damage exceeding $10 million and "inestimable" utility damage.

Deputy state fire marshal Tom Nickle said the explosions involved 150 to 200 gallons of hexane -- a volatile liquid hydrocarbon -- in the sewer, and the Metropolitan Sewer District was investigating the possibility that the hexane might have leaked into the sewer from a Ralston-Purina Co. soybean mill.

The sewer department said Louisville's primary sewage treatment plant was being bypassed to avoid possible damage, and untreated sewage -- more than 60 million gallons daily -- was being dumped directly into the Ohio River, used as drinking water source by downriver communities.

The explosions damaged streets and sewers in an area 15 blocks wide and 14 blocks long, knocked people out of bed, ripped gaping holes in the street and tossed manhole covers in the air.

A series of minor explosions occurred three hours after the original blasts up to 10 blocks from the scene. Four schools were closed and the blast area was evacuated. The order was lifted after workers flushed the sewers.