One thing that shocked me was how accurately scientists were able to measure the movement of tectonic plates, down to distances on the order of tenths of a millimeter. The part that's even more impressive? It's done via GPS satellites, 20,000 kilometers above. I'm by no means an expert on GPS, but it seems like we'd have to know the speed and position of each GPS satellite down to absurd levels of precision, with respect to some non-moving origin (and considering tectonic plates are always moving, that's pretty hard). Even more crazy is the fact that the atmosphere between the GPS satellite and the Earth is always moving as well, distorting the apparent distance and requiring a detailed model of airflows, as well as clocks in both the satellite and ground station that are accurate down to the amount of time it takes light to travel 0.1 mm! I'm astonished it's possible to measure this accurately, and it's a testament to both how accurate our equipment is and how low we've gotten the uncertainty in these measurements to make that 0.1 mm meaningful.