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Update 31.12.17: Finally, after years of collecting pumpkin, the floor is now finished as it should be!

What is this thing?
It's an automatic, AFKable Guardian farm. Quickly said, not quickly done. This is definitely one of my largest projects ever - not only in terms of how long it took but also in terms of how resource-heavy it was.

How does it work?
As Guardians are a water mob, the spawning area is a 20x20x20 'aquarium' which means there are 8000 spawning spaces which are easily enough for this farm to sit on the mob cap all the time, even during the night. One important thing to keep in mind is that a Guardian spawn will fail 95% of the time if the spawning block isn't covered by a solid block so the roof must be out of solid blocks.

The water is flowing downwards, so the Guardians are pushed down by it, fall 2 blocks and then enter a 4 block lava blade. The lava here is also flowing downwards. The burning Guardians then fall into cobwebs to stop them from jumping all over the place and die almost instantly after they've been caught. The loot they drop is caught by hoppers, go through a super-fast item elevator and are sorted and stored automatically. Due to the insane amount of items this farm produces I built 2 separate storage rooms and even this is only barely enough to keep up.

Guardians might be a water mob, but unlike Squid, they are treated as a normal, hostile mob, so you do need to light up the surrounding area. But as we are in the middle of an ocean there aren't many caves around so you should be ok.

The build
Needless to say, this thing was a handful to build. It started with finding an ocean monument, which was challenging enough because this world was created before they were added. I ended up finding 2 quite close together so that was cool. Clearing the water was a nightmare, but hey, what else was there to expect? I used a combination of all the Sponge I found in the 2 temples (one of them didn't have a sponge room so that didn't help much) and spamming dirt in the tighter places. I had to do most of this on Peaceful difficulty as it was just too nasty out there. When that was finally done it was just a question of taking out the monument.

Sticking with my style of building the mob farms in this world in the colors of the mob it.... farms I went with orange and cyan. The dyes aren't too difficult to make, it was getting all the sand for the glass was the larger pain here. My iron supply was plundered too as I used over a double chest of it for all the hoppers, but by far the most annoying part was the lava blade, so annoying I'm going to run out of words to describe how bad it was :P

The floor is out of carpet so that I could hide jack-o's underneath it to light the place up (like I did in The Nexus) - and also to stop Slimes from swarming the place. Another unique thing about this build is the 'hanging' roof with the arches from which I took inspiration from a building in my country where they store nuclear waste (and no I don't live in North Korea) and to get up to the roof and enter/exit the farm I also added an elevator.