If you saw a massive, bizarre-looking, rusty dome traveling along a major highway and are wondering what the heck that is — yeah, we were curious, too.

Turns out the gigantic piece of mystery equipment, which puzzled Southern California drivers from Corona to Norco to Westminster and Seal Beach early Thursday, is a reactor bound for the Chevron refinery in El Segundo.

It’s been on the road for days from a specialty manufacturer in Tulsa, Oklahoma, and made one last rest stop in Garden Grove on Thursday before its final journey, said Chevron spokesman Rod Spackman.

The reactor head was due to leave Garden Grove at 9 p.m. and wind through Long Beach and into the South Bay before its expected arrival at the refinery by 5 a.m. Friday.

“We have some scheduled equipment replacement that’s going to happen here later in the year and part of what is being delivered is a reactor portion for what is called our FCC, or fluid catalytic cracking unit,” Spackman said.

Eventually, the reactor will go up several hundred feet on top of the unit, which refines crude oil into gasoline.

“It’s part of routine maintenance, we don’t do it very often,” Spackman said.

Typically, he said, Caltrans and the California Highway Patrol coordinate massive deliveries late at night on routes that can handle the haul’s profile.

In the case of the FCC reactor top, it will zigzag via surface streets through Long Beach and the Harbor Area, enter Lomita on Pacific Coast Highway, then turn on Anza Avenue to Torrance Boulevard in Torrance before taking Sepulveda Boulevard to the refinery, Spackman said.

Chevron wasn’t quite expecting the reactor to generate so much public attention.

The move is nothing like a major one that attracted spectators for three days in February 2013, when a stretch of Sepulveda Boulevard was shut down overnight to bring six 100-foot-long steel coke drums to the refinery, attracting spectators.

On Facebook, motorists speculated about what the reactor head might be, with one guessing it was a World War II relic.

“We really didn’t appreciate probably that this would garner that level of interest,” Spackman said.