Take a photosensitive glass optic fiber slightly thicker than a human hair. It's incredibly lightweight, flexible, and stronger than steel. It's also incredibly cheap, at $10 a kilometer.
Now, pick a section of it, and zap it side-on with a UV laser until it discolours slightly. Move sideways a very short distance, then zap it again. Let's say the zapped parts are the same distance apart as the wavelength of blue light. Now shine white light down the fiber. Most of the light will go along the fiber, but the blue light will bounce back from the zapped section.
Now heat up the zapped bit of glass fiber. It will expand, and the zapped bands will move apart slightly. Instead of reflecting blue light, it will reflect blueish-green light (which has a longer wavelength). By measuring the change in reflected colour you get when you shine white light down the glass fiber, you can tell the exact temperature.
Now glue the zapped bit of glass fiber to something like a chair. Now sit on the chair. The glass fiber will stretch or shrink when the chair does, and again the colour of the reflected light will change. By measuring the change in reflected colour when you shine white light down the glass fiber, you can tell the exact strain that the chair is experiencing.
Now, talk next to the chair. The chair will stretch and compress slightly when sound waves hit it. This will make the glass fiber stretch and compress slightly, and the colour of the reflected light will change at the same frequency as the acoustic waves from your voice. You can now use the chair as a microphone.
Now, zap hundreds or thousands of sections of the glass fiber. In each section, make the zaps a different distance apart. Each section will now reflect a slightly different colour. You can now measure the temperature, strain, or sound at each one of those sections. You only need a single control box to shine light down the glass fiber and read the reflections, and you can record data from hundreds or thousands of points.
Now, string that cheap fiber across a city at the same time as you are putting up phone lines or power cables. You can now cheaply measure the temperature at thousands of points across dozens of kilometers, for only pennies per sensor. Great for weather forecasting! Or you can use that as thousands of sensitive microphones, listening in on conversations everywhere.