One of the things people are scared by these experiments is that they have the capabilities to produce micro black holes. No, these won't swallow us, but they do tie in with a rather terrifying theory on the quantum level. Let me explain (to my non theoretical physicist's understanding);

As far as we know, our universe exists as a part of a perturbative vacuum- that is, the lowest energy threshold attainable at a quantum level. Our laws of physics, fundamental forces and makeup of matter are based on this vacuum state.

There are three potential attributes to this vacuum state. Depending in part on the mass of the Higgs particle and top quark, it could either be stable, metastable or unstable.

The best scenario is that the universe is in a stable vacuum state. In this scenario, the zero point energy (read; vacuum) of the universe is the true lowest energy state of the universe, and nothing will ever make it descend to a lower energy state- it will persist with its current laws and forces until, perhaps, the extremely far future (quantum forces will likely become more prevalent around the time of heat death).

The second scenario is that the universe is a metastable "bubble" of a vacuum value higher than the true vacuum energy. Such a scenario might arise if the universe had some sort of quantum feature (read: barrier) which prevented it from rolling to this true zero point energy- thus- it persists, precariously, onward in the state we know and love. However, this metastable state could be undone through the creation of very high energy particles (much higher energies than humanity can produce) or the creation of micro black holes (due to quantum tunnelling hoo-hah space magic and vacuum decay around black holes).

The third, unlikely, scenario is that the universe is unstable and could collapse into the true vacuum at any moment. Given the age of the universe and the fact that this hasn't happened spontaneously yet, this is unlikely to be the case.

Now, back to the point, the micro black holes produced at the LHC, through crazy quantum mechanics I don't fully understand, could potentially trigger a collapse of the vacuum- provided it is metastable and not stable- into the true vacuum. They'd somehow overcome the barrier holding the universe in its current state and a singular point would classically tunnel into the true vacuum's lower energy state.

A bubble would be nucleated within which all the fundamental fields have hold their true vacuum values. This bubble would expand at close to the speed of light. Any area of space it consumes would have all field values instantly converted to their true vacuum values.

In effect, any volume that is swallowed by the bubble would instantly be reorganised, and all our notions of chemistry and physics would be completely erased- new physical laws would exist within the bubble, rendering any structure natural to our current-vacuum-value universe destroyed.

Basically, this bubble would destroy anything it engulfed and would expand throughout the entire universe at light speed. If a vacuum metastability event were triggered at the LHC, we'd all be dead within a moment's notice.

Food for thought. If this metastability event had been triggered anywhere in the universe (perhaps due to a primordial black hole on the edge of it) the resulting bubble would already be racing toward us at light speed, destroying all it consumes. There'd be no warning, as the light emanating from it couldn't reach us before the light of objects it has already destroyed. We'd never know it was coming, and we could all be engulfed by it in the blink of an eye.

Meep

Like I said, I'm not a theoretical physicist by profession. If there are any out there, feel free to correct any science blasphemy I've written here today. If anybody knows the exact proposed mechanics behind vacuum decay in black holes, I'd be interested to learn from you!