This can have some impressive effects if the bottom of the world is all sand, as seen here: .
I strongly suspect this is due to the usage of trim and sealevel, causing the code that calculates the bottom to be off by one somewhere.
And I was right! When the value for y is smaller than the column generated by the terrain code, a negative amount of end stone is placed and bedrock is tacked on at the bottom.
I will fix this by moving the bedrock placement to outside the while core loop in terrain.py.
Fixes #10 by moving bedrock placement out of the while loop
The while loop allowed for bedrock to be overwritten so I
rewrote it putting bedrock in place before the loop begins
and adding a condition to keep it from being overwritten.