As someone else said, there are some difficult ones that require climbing gear (which reminds me of playing various MMOs) - one in my town is about 15m up a large tree!

Pretty sure it is actually just the longest international border in the world. I guess Chile and Argentina might be close, maybe Russia and somewhere, but USA Canada I reckon is the longest.

I teach at a secondary school in the UK, in a classroom with no external windows (but with air quality monitors). After 1 hour of 30 teenagers the co2 will be at around 2000-2500ppm which I can confirm is stuffy. Highest I’ve seen is in the next door classroom which made it up to 3800ppm back in the summer.

It really does make you (and the kids) feel really dopey, so not exactly ideal.

Not sure if you mean per year but mortgages are generally going to be over much longer time periods. A couple who I know are looking to buy somewhere new and are looking at getting £400k mortgage or thereabouts. With rates as they are now, and over 25 years, they’ll end up paying back £900k!