time-5
Explanation
The Joke
A character (resembling a mad scientist) is caught using a time machine again. He explains he's booking a huge stone monument in the geological past to match twentieth-century iconography so he can maliciously mislead future archaeologists. In the next panel, set in the past, he is gleefully carving a modern-looking monument while declaring it will be consigned "to the basement of unplannedness."
The Humor
The comic plays on the idea of using time travel for the most petty and academic purpose imaginable: planting anachronistic artifacts in the deep past purely to confuse future scholars. Instead of using a time machine to change history, prevent disasters, or gain wealth — the standard time-travel plot devices — this character uses it for an elaborate archaeological prank.
The humor also touches on real debates in archaeology and history about anomalous artifacts (sometimes called "out-of-place artifacts" or OOPArts) that conspiracy theorists seize upon as evidence of lost civilizations or alien contact. The comic suggests a more mundane explanation: maybe some jerk with a time machine just put them there to mess with people. It's a characteristically SMBC combination of nerd knowledge and absurd misuse of extraordinary power.