This has happened twice now in the same world in different places. The first time it happened my game crashed before I went to look at it and when I loaded back up it had gone, but this time I captured it. As you can see in the screenshot, this is quite odd. The first time it happened it was in desert and so was all sand.

I dont think it was there all the time. it MIGHT have appeared when I went into creative mode to get a better view of my town - I hadn't noticed it before that.

edit: I just investigated the 'structure' and when floating under one of the blocks (in creative) it randomly teleports you to the next block above. when standing in a gap at the bottom of it I would teleport one block to the side while looking around

EDIT EDIT: in survival when standing under the structure you take damage as if suffocating - the bubbles appear but dont go down as well. also, some teleporting

EDIT EDIT EDIT: It happened again! same world, again a slightly different place. once again I think it happens when you enter creative mode. This time its a tree (see screenshot). This time, again the game crashed almost immediately after I saw it. However, when I started the game again, it was still there. I closed the game manually (not just the world, the whole game) and restarted and again it was still there.

I have closed and restarted three times now (both in creative and survival to see if that makes a difference) and this time it is still there

EDIT EDIT EDIT EDIT: This is the oddest one yet!!!!! I have walked around an area placing stone blocks to mark out a wall. I then started going around again with my wall block placing the lowest level, and on this go around I saw it. I haven't been in creative for a while, so its nothing to do with going in and out of creative it seems. 

this time, as you can see, it has formed from sand/sandstone and WATER! there are no breathing bubbles when you stand in the water, suggesting its not there. Yet when I broke the one made from a tree, I got blocks from it, so it MUST be there. 

Aliens? 

EDIT AGAIN: Its still there in the 0.50 update. This time with sandstone, sand and lava. the lave slows you down when you walk through it, yet does no damage. so its there and not there at the same time

AND AGAIN: just logged on (0.51) and there it was again. this one has broken a cactus, so even though its not there, its there, if you see what I mean