This achievement is story related, so unmissable if you keep playing.
*** Spoiler - click to reveal ***To unlock it, you'll meet a guy named Michael in the sewers. The plan is to detonate 4 explosives in a particular pattern, to show the world there are still humans in the quarantine zone. To do that, you have to reach out to apartments 105, 106, 134 and 137. Plant the explosives on the ovens and get back to where you left Michael. After that, a cut scene will start and you'll see how you detonate the explosives. Achievement pops right after.