Optical fiber spans the entire globe. Submarine cables, in particular, cover the seafloor; an extremely remote and harsh environment where humans cannot monitor for instance by placing temperature sensors and seismic sensors on the ocean floor.  Researchers have been able to show that with extremely stable lasers, they can use submarine cables to track earthquake waves propagating around the world along the cable length  potentially providing Tsunami early warning system. Other potential aspects are deep ocean temperature monitoring for climate change.  To do this on a mass scale will require the gigantic ultra stable laser to be miniaturized and mass produced along with the interrogator.  I believe this is a very interesting topic, and ties PICs to environmental and climate monitoring :)