DCS, was discovered in the 19th century by caisson workers - bridge, dam, boat or pier construction or repairs would have an airtight working area created (a caisson) around the foundations/work area...
It depends. You might get it or not. Some people are never altitude sick. Some are, when they are already on the altitude of few meters from the ground.
The lowest recorded case of serious altitude sickness (HAPE) was at 2500m in a climber in Japan. However, most people need to travel above 3000m and stay there for at least 12 hours before symptoms...