In this chapter, we discussed the atmosphere in terms of pressure, density, temp

In this chapter, we discussed the atmosphere in terms of pressure, density, temperature, composition, and the various layers. I’m confused about a couple things. Please explain them to me:
At 18,000 feet, atmospheric pressure is about half of what it is at sea level. Thus, I would suspect that at 36,000 feet atmospheric pressure would be zero. However, this isn’t the case. Atmospheric pressure doesn’t reach zero until infinity; in other words, an infinite number of feet above sea level. That’s a lot farther than 36,000. How is this possible?
If I go scuba diving and descend to only 33 feet below the surface, I experience twice the atmospheric pressure as compared to standing on the ground at sea level. How is this possible?