Brain of Cthulhu
The ground the mimic spawns on is 40m above, so I think we can assume that metric & imperial have different altitudes for 0.
Yes, this is interesting. You should grab a depth meter, go to 0 altitude, and post a screenshot.
Is there a reason the blocks are staggered, instead of straight rows 2 blocks high?
The blocks are staggered to prevent any 3-tile-wide vegetation from growing. If too much of such vegetation is allowed to grow then actuating the ground will not guarantee breaking the plantera bulb.