According to local station KTLA, it's one of the largest water main breaks ever seen. From the article:
A large water main break in Westwood sent water flooding into streets, stranding people and vehicles and prompting the closure of Sunset Boulevard on Tuesday.
People were stranded by rising water, according to the Los Angeles Fire Department, which was called the incident shortly before 3:30 p.m.
Sunset was closed from Marymount Place to Westwood Plaza, according to LAFD.
The Los Angeles Department of Water and Power was en route, according to the Fire Department.
The address given for the flooding was 10630 W. Sunset Blvd., next to the UCLA campus (map).
“Most affected is ‘UCLA side’ of this apparent water main rupture,” the Fire Department’s Brian Humphrey said in an update.
KTLA has a live feed of the activity at the scene.
Here's a screenshot of the underground parking lot entrances:
Near the steps to Pauley Pavilion: