Fluids can definitely go into built cages, at least; I've flooded a room with some goblin cages in it and noted they drowned after a while. One time I accidentally had pressurized magma overflow into the area where I was storing goblins waiting to be thrown into the magma, and tons of smoke came out of the (iron) cages, followed by XXgoblin bonesXX being swept along with the magma currents.
I don't know if they'll go into cages in stockpiles.
Sinergistic is confused, and should replace all instances of "terrarium" in his post with "aquarium" and then "glass cage" with "terrarium" to make his post correct.
Just one point I'm curious about:
Sir_Geo, you're saying that in your experience, loading a cage trap with a terrarium and then having it triggered while underwater will cause the water to be captured along with the creature, so that a captured fish will not drown in an aboveground stockpile.
When this happens, does it still get called a terrarium, or does its name change to an aquarium? In other words, does the terrarium/aquarium terminology refer to whether or not it actually has water in it, or just how it's been designated?
If it stays a terrarium, and then you build your carp terrarium, does the fact that it hasn't been designated as an aquarium mean that dwarves will dump the water out of it once it's built, thereby drowning the carp?
And if that's the case, what happens if you capture a goblin in a terrarium, then build it and designate it as an aquarium?