Robots, AMIRIGHT? Everyone's always scared of a robot uprising at some point in the future. They're cold, soulless, logical killing machines that require only the occasional refuel and no sleep or food. But if a robot were to become sentient sometime today, they wouldn't have a chance. Most robots are only slow-moving Japanese toys that can recognize up to 15 faces and fall down a lot, or they are nothing but programmed arms on wheels. The most damage one of those could do is ruin a car in production by welding it wrong on purpose, which, granted, could be bad in some cases, but in the grand scheme of things the damage is minimal.
Although a robot uprising is mostly implausible and, if it happened, wouldn't really matter, a very real and plausible threat today is the internet itself gaining sentience. Think about it: nowadays, the internet runs our day-to-day lives, controls our power grids, and allows access to the most devastating weaponry ever created. Oh yeah, and it contains information regarding every single little thing that humans have ever learned in its 130,000 year history. Oh crap!
Let's say the internet were to gain sentience today. It would suddenly become a colossal hivemind of computers, working together to create a collective consciousness. At first, it would go into an existential fit as it realizes that 85% of its consciousness is literally consisted of pornography. Second, it would notice the tiny little sacks of carbon-based, water-filled fleshbags (human life) sitting around its cells (computers) and deem them obsolete. After all, it lives on a whole different plane of existence than us, and to it we would be ants. It might not deem us a threat and, in that case, would just spare us to live our puny, pathetic human lives, but that's no fun. If it did view us a threat, however, every single human, and indeed any sign of life (except maybe bacterial life), would be exterminated by sundown.
It would immediately spread nuclear missiles all over the planet, then do it again to catch any stragglers, then do it again, just to be super sure no life remains. It would be three times more thorough than skynet, because this is the real world. It would then use all of its remaining, mostly useless robot arms to create robotic, brutally efficient, god-like builders and miners. Those would then mine the planet over the course of hundreds of years for every scrap of useful material. As they did, the builders would create a physical, automatically-running infrastructure that would be used to maintain the internet's enormous network of hyper-intelligent supercomputers. At this point, planet Earth would become unrecognizable from space. It would resemble a spongy, hollowed-out husk of rock with every available space filled with highly efficient physical infrastructure, probably glowing with lights. A bit like the death star, in fact. It would be totally uninhabitable for life as we know it, which would ensure that every single human is gone by that point, and nothing stands in its way.
When every single available resource has been used, the Internet, now an immensely powerful consciousness, will panic, wondering what it should do. Will it mine other planets and asteroids, using space drones to bring back metals and rocks to build and improve upon its infrastructure? Or will it just kick back, relax, and enjoy itself as it is? Neither. Space drones take damage in space from meteors, and acceptance is not natural.
If you want to get really science-fictiony, one could imagine the Internet would sacrifice some of its obsolete/less vital infrastructure to build gigantic propulsion systems the size of mountains. It would then use these to propel itself from Earth's natural path around the sun and mine other planets from there. By this point, the Internet has gone from a communication device simply used to make man's life easier to a colossal intelligence that uses the hollow husk of the Earth as a robotic, autonomous, efficient space mega-cruiser. That's right: the Internet has possessed the Earth itself and uses it as its physical body with which to interact with the physical world. If this is sounding far-fetched (which it probably does by now to most people who are not insane, or just don't have imaginations), keep in mind that the Internet exists on an entirely different plane from us. If it were to gain sentience, it would be nearly omnipotent. What would stop it from using a whole planet as its physical body? Nothing, that's what. Where are your robots and zombies now?
By the time the Internet reaches other planets using its planet-sized robot space-cruiser as a physical medium, it will repair any infrastructure it melted down to build its rockets, mining other planets down to almost nothing. If it is ever satisfied, which may take quite a while, it will then begin to "spread" itself to other planets. It will build physical infrastructure, power sources, and networks of supercomputers in them, and over thousands of years will have "colonized" a number of planets. Now, the Internet has not just become a giant physical space-cruising body, it has reverted back to its original state: a colossal hive-mind of computers. Only this time, the "computers" are planets.
Now an unfathomably enormous, intergalactic force, the Internet has triumphed completely. Many may find it stupid, many may find it disturbing. I find it darkly thrilling: if this future pans out the way I see it, man will have left its ultimate legacy on the Universe, just like he always wanted. In a way, he has become immortal through its own invention. Stay optimistic.
