Microwaving a glass of water, what happens? Most of us here would already know the simplified idea behind microwaving food: Microwave radiation hits the water molecules present in food, which excites them and causes 'em to vibrate rapidly in situ resulting in the heating up of food. Very straightforward.

But what if you decide to microwave a glass of water?

I can't seem to find any reliable literature that deals with this. A lot of highly unreliable sites spout nonsense from "The water becomes poisonous" to "The water becomes radioactive", so I've immediately dismissed those. However, some sources suggest that the "water will explode".

Now I was under the impression that the water would perhaps come to a boil a bit faster than it would by simply lighting a fire underneath it. 

So after reading the last few sources it wasn't really difficult to believe that an explosion might occur. At first I simply extrapolated my theory of 'rapid boiling' to the theory of 'Very rapid boiling' and was content with that. It made sense to me since I presumed that, just as in the conventional method of boiling water, bubbles of water would be formed at the bottom of the glass albeit at a much faster rate, build up, rise, thus resulting in an explosion. But after some reflection, I noticed a problem with this 'extrapolation'.

Why do the bubbles even form in the first place when you conventionally heat water? Simply lighting a fire below a beaker of water does not ensure that the entire content o the beaker is uniformly heated at the same time. Obviously the layer of water molecules in direct contact with the bottom of the beaker will get heated rather quickly. They rise up, colder water descends, etc etc, you know the deal: convection. It's the sizable temperate difference between the bottom region and the rest of the water, that enables the water at the bottom to get vaporized at some point forming bubbles while the water immediately above it is still in the liquid state.

The problem I see that prevents me from extending this theory to microwaving water is that the mode of heating is different in the case of the microwave: Radiation

Now since radiation is faster at heating a given volume than convection is; for all practical purposes, I think it'd be safe to accept that the water in the beaker will be heated uniformly (The beaker's pretty normal sized...it's dimensions aren't of an order of magnitude greater than that of the speed of light/electromagnetic radiation, so it'll be alright to take the apparatus as being 'uniformly heated'. Had to say this before someone inevitably points out in the comments that all the water being heated to constant temperatures like this at the exact same time is impossible).

So if the beaker is heated uniformly, then I see no reason for bubbles to form...in other words, I don't see an explosion coming.

So what really happens if you microwave a glass of water? 

Will it quickly and steadily vaporize? Will it explode? Or does something else happen? And why does it happen?
Heating water on a hot plate is safe, because the hottest point is at the bottom of the pot. A lot of relatively small bubbles appear there without much overheating of the water, because there is a lot of nucleation at the uneven phase boundary steel-water.

In a microwave, the hottest place is IN the water. The glass does not get heated by microwave (at least not much), and radiates off some heat to the surrounding.

Problem: In clean water, there are few good nucleation points to form bubbles, only some dust particles perhaps. So the water gets overheated rather strongly, and a first bubble that appears can grow a lot before it has cooled its surroundings down to 100 °C. That one huge bubble can throw most of the water out of the glass. It boils over, violently.

Btw. a microwave does not heat its content uniformly. It forms a standing electromagnetic wave (that's not radiation, strictly speaking) in the oven, like a rope swung quickly between two people, or a guitar string. The wave pattern has knots at a distance of $0.5c/f\approx6\ \mathrm{cm}$ (with $f=2.45\ \mathrm{GHz}$ and speed of light $c$ which of course is a bit less in your chicken), where there is very little heating. That's why the microwave oven has the rotating plate, to generate some uniform heating. For pure water, that doesn't matter, because convection sets in anyway and distributes the heat.