Random Generated Structures

I and several others have encountered a randomly generated structure inside the same seed while attempting to share the seed to others
the seed number is -3103576636145444240, it appears at random in different locations and appears as what appears to be a new structure made with acacia wood planks and leads down to a powered rail that then leads to a room full of ore some time. Other times it appears a a void in the ground that you can fall to your death in, other times just a tunnel staight down with acacia wood planks scattered throughout.

What I expected to happen was...:
A normal world that stayed the same every time we used the seed.

What actually happened was...:
A world that has a random generated structure about 1 out of every 25 times.

Steps to Reproduce:
1. Input the seed.
2. play like normal
3. sometimes find random structure.