Arnold Schwarzenegger Pothole Was Service Trench, But Work Was Long Done

The L.A. "pothole" Arnold Schwarzenegger fixed was indeed a service trench for the gas company ... but TMZ has learned that service was done months ago, and that's why Arnold went all DIY on the ditch.

We got to the bottom of Arnold's roadwork adventure -- a story with more twists and turns than one of his action blockbusters.

ICYMI, Arnold rolled up his sleeves and went to work Tuesday ... using muscle, Quikrete and some tools to pave over what he saw as a "giant pothole." Frankly, he was fed up with the shoddy pavement that was wreaking havoc on tires for months in his Brentwood neighborhood.

Arnold posted his good deed on social media, and was mostly met with applause ... like the driver who stopped during the middle of his construction project to thank him.

But, the Los Angeles Department of Public Works came out and said the "giant pothole" was actually a service trench being used by SoCalGas to repair a pipeline. That's true, however, the gas co. work was wrapped up way back in January!

What's worse is the city had no plans to fill in the trench until May. That was way too long for Arnold and several other annoyed neighbors ... which is why he took matters into his own hands.

As Arnold explained ... "Today, after the whole neighborhood has been upset about this giant pothole that's been screwing up cars and bicycles for weeks, I went out with my team and fixed it. I always say, let's not complain, let's do something about it. Here you go."

The saga is not done yet ... the gas company reportedly sent crews Wednesday to level off the patch Arnold filled, in order to strengthen the paving mixture he used.