Mercury Delay Line Memory
A considerable amount of engineering was needed to maintain a clean signal inside the tube. Large transducers were used to generate a very tight beam of sound that would not touch the walls of the tube, and care had to be taken to eliminate reflections from the far end of the tubes. The tightness of the beam then required considerable tuning to make sure that both transducers were pointed directly at each other. Since the speed of sound changes with temperature, the tubes were heated in large ovens to keep them at a precise temperature. Other systems instead adjusted the computer clock rate according to the ambient temperature to achieve the same effect."
It is really interesting to me to see such high-tech solutions to problems that no longer even exist. A top-of-the-line memory module from the 40's with exciting carefully engineered features is completely outclassed by a cheap modern memory card. We live in a mundane age of science fiction.
The way I see it, though, that kind of thing still exists today: the "cheap modern memory card" probably contains many "exciting carefully engineered features". They got teams of people working full time to make these thing as fast, reliable, cheap, low-power as possible.
But the engineering feats are likely much more niche (and thus harder to understand). Not to mention, secret.