Several of the suggestions have involved "spike fields" or massive blocks of concrete or granite, rendering an area inhospitable, forbidding, and worthless for agriculture or human habitation. Teams from Sandia labs also came up with several "messages" that they wanted the imagery to convey, two of which are condensed into these patches. It's pretty fascinating stuff, and you should look into it. Here's the Wikipedia page to get you started.
Of course, humans being humans and idiots, these warning messages may perversely encourage exploration of these dangerous sites. Pharaoh's curse and all that.
These stickers are 3.5" across, and available in two versions as seen above. The "both" option comes with one of each. Also available as patches here, or as a combination bundle with the patch and sticker together here.
Art by Jade @JadedPencil