Transparent walls - well, walls are transparent to many types of electromagnetic waves, which can be visualized: https://www.sciencealert.com/wi-fi-signals-can

Transparent walls - well, walls are transparent to many types of electromagnetic waves, which can be visualized: https://www.sciencealert.com/wi-fi-signals-can-identify-you-through-walls-and-even-track-your-movements https://www.sciencealert.com/wi-fi-signals-can-identify-you-... https://www.nbcnews.com/tech/tech-news/mit-device-can-detect-people-through-walls-n452926 https://www.nbcnews.com/tech/tech-news/mit-device-can-detect...