They're geoglyphs, which are symbols encoded in the earth.

Idk why this is getting down voted, the article identifies them as geoglyphs in the second paragraph.

	▲	alephnerd 4 minutes ago \| parent [-]
		Because of misplaced pedantry by a couple ESLs. I've noticed that Europeans users now comment at a higher frequency on HN than American users now, and set the tone of the site because of timezones.