sinkhole San Francisco
© Jeff Chiu/Associated PressEmergency crews work on removing a big rig truck stuck in a sinkhole in San Francisco, May 5, 2017.
Crews found a 3-foot-deep void where a San Francisco street caved under a heavy big-rig truck on Friday, but they are stumped as to what caused the hole.

Water is usually to blame, but crews could not find any water leaks, San Francisco Public Utilities Commission spokeswoman Betsy Rhodes said.

"The culprit to create a void like that is water, but the trench is dry," she said.

The massive sinkhole in the city's South of Market neighborhood developed early Friday after a heavy big-rig parked on what Rhodes said was essentially asphalt over air.

The driver escaped unharmed through the driver side when his big-rig started sinking on the passenger side. Driver Alejandro Curiel told KRON he couldn't believe what happened.

Rhodes said the sinkhole measured 3 feet deep (0.91 meter) and 10 feet (3 meters) by 20 feet (6 meters). Crews are covering the hole with sand and gravel.

Someone started a Twitter account in the name of the sinkhole and said it was just chilling. It also wished everyone a happy #Sinkholedemayo in honor of Cinco de Mayo.