Two out of nine seriously injured firefighters remain in critical condition in the hospital -- this after a compressed natural gas tank on a burning truck exploded Thursday.

You gotta see this wild video ... dramatic dash cam footage from Wilmington, CA shows the exact moment the fiery blast ignites and sends a fireball up into the air. You can practically feel the intensity just watching it!

As you can see, the explosion launches at least one firefighter into the air among various other car parts. You can also see the ground shaking like crazy when the blast goes off in another angle.

Here's what happened to lead to the actual explosion itself ... it was around 7 AM when firefighters responded to a call about a burning truck on the road -- when, suddenly, one of two 100-gallon compressed natural gas tanks on the truck blew up minutes later.

Los Angeles Fire Department Chief Kristin Crowley has revealed one of the critically injured firefighters is stable ... but is still being monitored at Harbor-UCLA Medical Center.

The update on the second critically injured firefighter isn't as promising ... he was intubated and transferred to L.A. General Medical Center to be treated at their burn unit.

The other seven firefighters aren't out of the woods just yet either ... according to LAFD Capt. Erik Scott, they're still going to need ongoing treatment for a laundry list of injuries – burns, blunt trauma, shrapnel wounds, and even damage to their hearing.