A spider spawner built in the center of an Ancient City

Apparently, the generation rate is between 0.00000000000004% ~ 0.000000000000000001%.

If you go to these coordinates, you’ll find: ・Iron vein ・Copper vein ・Abandoned mine ・Zombie and skeleton spawners (there are two chests next to each other, but they haven’t merged into a large chest) ・2 Trial Chambers ・2 Ancient Cities ・A spider spawner built in the center of an Ancient City ・An End Fortress you’ll be able to see all of these structures.