Vaccines kinda undercut that.

Both. There are quadrillions of tons of atmosphere on Earth and it's composition doesn't change over night. At the same time, most plants and algae will absorb CO2 and release oxygen. They'd also absorb oxygen but most plants need the extra carbon more than they need the spare oxygen. Mind, this only works so long as we aren't destroying plants and algae while simultaneously dumping CO2 into the atmosphere.

People either died or became immune. Eventually, plagues burn themselves out, running out of new people to infect and being eliminated in the bodies of both the survivors and the victims.

Mostly it's to make pills easier to identify. You don't want to mix up your anti-allergy medicine with your antibiotics after all. There's actually entire dictionaries of pill designs so that doctors and pharmacists can identify pills at a glance.

Not what I was saying and not accurate. Change the direction of a barometer and the reading will stay the same. Pressure might increase with depth but the pressure hits you from all directions at once.

First off, we were born into it. Honestly, if we didn't have about 14 pounds per square inch pressing down on us we'd suffer and possibly die. That pressure keeps our blood from boiling and oxygen from leeching out of our lungs. Our bodies exert about that much pressure on the air around us and that keeps things balanced.

Second, air pressure hits us from all directions equally. Yes, the air is pushing you down but the air underneath you is pushing you up. You're being pushing forwards, back, left, and right all in about equal measure so the net effect is just a gentle squeeze rather than a crushing force.

It's possible that the rocks were there first and repurposed as boundary markers afterwards. You often see rivers and streams used this way.

That being said, it's entirely possible that someone deliberately used an oversized rock to mark the boundary because they didn't want something that could easily be moved. Using stones is an old method, though, dating back to antiquity.

They'd literally hammer numbered stakes into the ground , hence "Stake your claim". The exact extend of the claim was usually defined by distance from the stake and farmers would usually fence off the extent of the claim as soon as they could.