Regarding engineering achievements of the 20th century that had the greatest impact on society, impact can be good or bad. Many were killed in other wars and 107 million were killed by weapons of war in countries where there were no wars, in the most brutal century of all. Agricultural mechanization has kept food prices low with increased productivity, but has had a devastating impact by eliminating small farms and villages. The space and missile programs were not that great for engineers. While some were lounging in comfortable chairs watching the moon walk, engineers were being laid off just short of the 10 years required for pension vesting.
Water is the most important substance on earth.
Rain and snow melt must be stored and then released at uniform flow rates to sustain life in places like New York, Los Angeles, Las Vegas, and Phoenix, as well as to irrigate farm areas. Releases allow generation of electricity. Storage reduces or eliminates floods. The New York City water supply system was designed and built just after 1900. The water flows by gravity and no pumps are needed.