Superman Vs The Thing From 40,000 AD

Superman, being from Krypton, was an alien himself, but more often enough found himself protecting Earth from other alien invaders...

...but, there were times he also protected Earth from invaders from other times, as he did when he faced the Thing from 40,000 A.D. (covers by Wayne Boring/Stan Kaye and Denys Cowan/Bob Smith.

The Thing From 40,000 A.D.!

In Superman #87 (February, 1954) by Bill Woolfolk, Wayne Boring and Stan Kaye, Superman found himself in a jam, as a strange protoplasmic creature landed outside of Metropolis.  The Thing quickly adapted to the land, copying what it found.  Clark Kent went to investigate as the U.S. Army was as well, and, as Superman, stopped a truck full of explosives from going off (as the Thing set it on fire to distract the army from his impersonation of one of its members).

As the Thing pillaged scientific labs the area around Metropolis, Superman was following, finding it and its lair (where it was building a time machine to go back to where he came from to conquer Earth).  Superman confronted The Thing, first as Clark Kent, then as Superman, tricking it into an area where a H-Bomb test was to happen...and, leaving it there when the H-Bomb went off.  

Finding no trace of the Thing, Superman assumed it was dead. 

Metropolis Wasn't Build In A Day...Or Was It?

Starting off on the planet Kuraq, as a meteor with a familiar looking red and blue figure crashes on the planet is DC Comics Presents #89 (January, 1986) by Bob Rozakis, Todd Klein, Alex Saviuk and Ricardo Villagran.  On that planet were members of the Omega Men (an alliance of various species from the Vegan System, here...cat like Tigorr, bird like Harpis and newcomer Oho-Besh), who were enjoying the success in finding a home from refugees, when they were interrupted by the crashing meteor and a whirlwind of activity, building a stone replica of Metropolis!

Here, the aliens found themselves facing what they thought was Superman, as he oddly went through the motions he did as he faced the Thing, with the Omega Men taking the place of the villains.  This continued all the way to the site of the H-Bomb test....where Superman was incinerated.  Or was he?  The real Superman showed up (as the Omega Men had called for him) and revealed that was the Thing (whom Superman thought was dead) living Superman's life at the time.  Superman and the Omega Men left this planet as it was too strange (and didn't know that only the Thing's body was destroyed, and its essence stayed with the energy beings who populated this planet).