Their mission was called Operation Ogre, and it was completely insane. We're not kidding here -- Christopher Nolan would have scrapped it for being unnecessarily convoluted.
First, the commandos had to pose as students of sculpture in order to rent a basement apartment on a street Carrero Blanco regularly traveled on his way to Mass. The fact that these hardened warriors ever managed to convince a landlord that they were art students was a notable achievement in itself -- but it was just the beginning.
"Those are ... candles. Art candles."
Next, they started digging a tunnel under the road by hand, a task made all the more arduous by the fact that none of them had a clue about tunneling. Or anything else, really -- during the year that went into the preparations, countless nearly botched supply robberies, irrelevant arms raids and unnecessary maneuvers made it clear that as good as the team might've been at commandoing, they absolutely sucked rodent ass at everything else.
The second they started their tunneling project, they discovered that their pickaxes were far too large and it was impossible to swing them with any force. Then, one of the assassins remembered that he was claustrophobic, something that probably should have occurred to him at the "let's dig a small, dark tunnel" part of the planning stage. Then, the tunnel roof started repeatedly collapsing on them. At that point, the only thing that was missing was the soil being contaminated with, say, leaking sewage and noxious gases. Wait, it was? Never mind.
Noxious gases had long been one of Carrero Blanco's staunch allies.
Finally, someone had the bright idea to actually read a book on how to dig a tunnel. They started to use props, which stopped the roof-falls, but did little about the smelly sewage and gas, which by now was messing with their health pretty big time. Yet the men persevered, and one day, the tunnel was finally complete. That was the second most rewarding day of their lives.
The most rewarding day of their lives came soon afterward, when their target's car drove over the tunnel they had now filled to the brim with explosives. Their giant tunnel-bomb was a success beyond belief: It hurled Carrero Blanco's car five stories into the air. It just flat out flew over a church and to the other side, where it landed on a second story terrace:
"Damn. It was 50 points for the tall roof."
The men, disguised as electricians, looked in awe and did their level best to suppress high-fives. And justifiably so -- hell, we'd dig a tunnel for a year, if the reward was a badass GTA flip like that. We probably wouldn't put a prime minister in the car, though.
For more crazy assassination attempts, check out The 6 Most Utterly Insane Attempts to Kill a US President and The 5 Most Ridiculous Assassination Plots Ever Attempted.