UCLA Athletic Center -- COMPLETELY FLOODED ... In Massive Water Main Break

UCLA is virtually underwater ... the result of a massive water main break on campus -- and the athletic training center is getting hit HARD.

TMZ Sports obtained footage ... showing the water rushing in beside a mural that features several UCLA athletic greats.

The water main ruptured around 4pm ... and water is shooting out like a volcano.

In fact, it's so bad officials shut down Sunset Blvd. and firefighters were helping stranded motorists.

Besides the athletic training center, the soccer field is also under water ... and we're told there's water all over the basketball court.

Meanwhile, all is calm at USC.