There is a little known hazard when heating water in a microwave that involves filtered water and very clean containers.  One should never heat water for more than 3 minutes on high in a microwave oven.  The time should be enough to bring water to a boil usually indicated by bubbling; however, not all boiling water heated in a microwave oven will bubble.

Bubbles come from impurities in the water itself or on the inner wall of the container.  Filtered water in very clean containers can lower the amount of impurities to below that which causes bubbling.  Without impurities, the heated water creates a surface tension under extreme pressure that cannot be broken by the heated water itself; but, by being punctured with an outside instrument (such as a fork).

If after 3 minutes, the water is not bubbling, do not assume it is not hot and continue to heat it longer for it may be hotter than you think, just not bubbling.  Continuing to heat the water will not cause it to bubble; but, will increase the pressure without breaking the surface tension and cause it to explode if the surface tension is broken by other means.

I did not believe this myself until I saw it demonstrated.  The super heated water actually hit the ceilling and spread out over a wide area when the surface area was punctured.  The exploding water will be hot enough to severely burn human flesh.

For a demonstration of what happens to super heated water in a cup, see  Super Heated Water.