Back in 2013 I had a singleplayer survival world, one day I came across this pyramid structure that I haven't been able to explain to this day. (I no longer have this world as the pc I played on broke and the world was unsalvageable, only these screenshots remain). I've asked other people I know who play mc and they haven't seen anything like this. I have also asked the minecraft forums a few times and no one has a proper explanation.
What is pictured is a small-scale pyramid structure in the middle of a snowy biome. It is hollow inside except for a single column of water coming from the ceiling, two zombie spawners and a chest. As you can see there are items in the chest that can't be stacked or spawn stacked, i.e. the gold helmets.
I was the only person who played mc on the pc and cheats were turned off.
Desert pyramids were in the game at this time but were very new.
if anyone can explain or has seen something like this please let me know!