I want to know what key invention would allow my civilization to travel down to and explore its world at sea level. This is similar to my previous question* but this time does not assume metals are in short supply so technology may develop as on our world.
This world is an alternate Earth. Similar size, mass and distance from its star as our Earth but with a much denser atmosphere. At sea level the pressure is around 3.2 bar, the temperature is around 50 degrees C and the humidity is 100%. At 5000m the pressure is around 2 bar, the temperature around 25 degrees C and the humidity ranges from 20-70%. The oxygen concentration in the atmosphere is around 15%.
The humans live on a very large plateau at 5000m and cannot descend to sea level unaided because of the high temperature and humidity. Assume that their technology progresses in a broadly similar fashion to ours. The humans there are physically just like us.
As this society develops from a primitive level to more modern technology what is likely to be the key invention that enables them to more fully explore their world? In cases where multiple answers are considered equivalent the answer using the earliest technology would be considered better.