Legacy?

@ViciousKangaroo can we not make this a competition? :/

Business is competition. ;)

@CraftyMyner :D it's time based, some crazy logic going on there, it should show the avatar but it might have messed up.

Haha okay, thanks for saving me from the awkward moment!

@ViciousKangaroo Make sure you get a screenshot of the explosion! ;)

Yup, you keep everything except your rank (ouch) and there's a 99% chance you won't get it back--you have to be a good admin or staff with high reputation.

@Laineyyz Nope it's not. I saw it again after a few hundreds blocks out and it's 2 floating fires this time but I did not manage to screenshot it in time before it disappeared

Next time you see one try shooting it with your bow. :P If it moves, it's a fireball. Probably from some command block experimenting.

That's some pretty hot lava... evaporating the water before it can even touch it!

@NutjobBob We're all to scared of you iwarriiori!

Here is cross-section of the trap

The sand is actually an entity hovering above the fences. This means that when you try to break a block you just hit the sand instead. You cant place any blocks because the entity is in the way. Ender pearls hit the sand sand entity and teleport you nowhere (I think). You cant jump out because of the slabs above.
It is actually possible possible to mine your way out if you stand on the very edge of a block, and face in one of the cardinal directions. However this is really difficult to do and will be impossible when taking damage ticks.

After much testing, I have come up with a recommendation for you and a conclusion.

First, you are right about ender pearls only teleporting you where you are, EXCEPT if they are aimed straight up and thrown. This lets them go through the slab roof. This can be fixed by adding another layer of blocks on the ceiling (corners too!).

As for the conclusion, this trap is inescapable assuming:

1. You remove space for chorus fruit teleportation and
2. The player cannot survive long enough to mine the wall and
3. The player cannot survive suffocation long enough to escape through the ceiling.

So just clog the whole place with obsidian and you should be good! (Actually not that hard tbh.)

@FishW Place down a 3 block high tower. The place a fence connecting to the side of the 1st block. Then place a piece of sand on the side on the side of the 3rd top block and it should fall on top of the fence as an entity.
I used to use it in some cool build designs, and i think that was how i did it.
Alternativly you could place 2sand blocks on top of the fence post then destroy the lower sand block.

Guess I'm still not up to date with the mechanics! Thanks for the overview.

How do you get the sand to remain an entity? Some 1.9 thing I don't know about?