It's bad enough that we humans must deal with monstrous creatures that originate from our own planet. But life gets really trying when we have to fend off monsters from outer space that have invaded Earth.

Indeed, sometimes the most nefarious attacks are the ones that begin before we even realize what's happening. Those are not the only type of unwelcome alien invasions, however, as the following clips illustrate.

The Faculty (1998)

They're so cute when they're young! Until they start biting, that is, as high school instructor Jon Stewart and his students -- including Elijah Wood, Josh Hartnett and Laura Harris -- soon learn. But the little ones are not the only dangers facing the world.

Undead (2003)

Creatures from outer space arrive via meteorites to a small town, transforming unlucky locals into the undead. The aliens have a larger plan in mind, but it's even more diabolical than the zombies that are left behind.

War of the Worlds (2005)

The invasion began long ago, but only now are the aliens making themselves known. They are not friendly, to say the least.

Invasion of the Body Snatchers (1978)

Responding to call for help, San Francisco health inspector Donald Sutherland arrives at a bathhouse owned by his friends, Jeff Goldblum and Veronica Cartwright, to investigate a pod that is not quite human. It's an early indicator of the huge threat that faces the city and, indeed, the world.

The Puppet Masters (1994)

In this clip, we get a look at the aliens that will soon take over a small town, a small step on a journey toward world domination. The creatures are beyond reason, and the humans appear to be helpless to stop them.

Species (1995)

Young Michelle Williams has escaped from confinement at a medical facility where she has lived her entire life. Little does she know that her human DNA was spliced with alien DNA, and that she is about to experience the quickest, and most extreme, puberty on record. Earth is not ready for what she will become in this very sneaky alien-invasion movie.