[UPDATE - apparently there were already some images of the Ultra stage design floating around, though none that had received as much attention as Martin's Instagram post as far was we know. So this isn't quite the leak we initially though it was.]
It looks like Martin Garrix just inadvertently leaked the Ultra main stage on his Instagram.
He made a post showing himself and the video/lighting team, presumably planning the production for his Ultra set - but one of the screens seems to have the general design of the stage shown. There are already a number of comments on social media indicating as much (though of course there's no way to verify that any of those people are reliable sources who could confirm this with certainty).
We've got a screenshot below, just in case the original post gets taken down.
Obviously this is just one image, and couldn't possibly capture the full glory of the Ultra main stage, but it does still take a bit of the impact away from the initial reveal. It won't ruin anyone's Ultra experience, but there are probably some production folks at Ultra that are pretty annoyed with Mr. Garrix right now. Unless this is a clever misdirection ploy, and Martin is in on it. We shall see.