graffiti on a cyber wall

With all the dramatic possible endings to life as we know it (meteors, asteroids, pandemics, global thermonuclear mutual destruction, global warming...make up your own) here is a doom and gloom I've never thought about.
 

The gray goo problem

The "... danger of molecular nanotechnology is the risk that self-replicating nanorobots capable of functioning autonomously in the natural environment could quickly convert that natural environment (e.g., “biomass”) into replicas of themselves (e.g., “nanomass”) on a global basis, a scenario usually referred to as the “gray goo problem” but perhaps more properly termed “global ecophagy.”

As Drexler first warned in Engines of Creation:

“Plants” with “leaves” no more efficient than today’s solar cells could out-compete real plants, crowding the biosphere with an inedible foliage. Tough omnivorous “bacteria” could out-compete real bacteria: They could spread like blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter of days. Dangerous replicators could easily be too tough, small, and rapidly spreading to stop–at least if we make no preparation. We have trouble enough controlling viruses and fruit flies.