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 and patches are 3.5" across, and available in two versions as seen above. The patches are hook-fastener backed. The individual options come with one patch and two stickers in the same design. The "both" option comes with one of each patch and two of each sticker. Also available as individual patches here, or individual stickers here.
Art by Jade @JadedPencil