Superheated Water In Microwave Can Explode

Water superheated in a microwave oven can indeed "blow-up" under certain conditions. That is, the superheated liquid can be explosively ejected from its container and potentially cause injury to a person in close proximity.

such events can take place when the following conditions are present (quoted material in green):

Using a container with a very smooth surface, such as an unscratched glass or glazed container.

Heating for too long.

Quickly adding a powder, such as instant coffee (or sometimes even an object to stir it).

Standing with one's face above the container makes injury more likely.

Superheating occurs when the liquid is heated to a temperature greater than its normal boiling point. Wolfe explains that the "superheated state is unstable, and it can very rapidly turn into liquid at the boiling point, plus a substantial quantity of vapour." Thankfully, some simple precautions can minimize the chance of injury due to superheating. The following strategy for avoiding superheating related injuries :

Before putting the water into the oven, insert a non-metal object with a surface that is not smooth. (e.g. a wooden stirrer. A wooden skewer or icecream stick will do.)

Use a container whose surface is at least a little scratched.

Do not heat for longer than the recommended time for the quantity of water used.

Tap the outside of the container a few times with a solid object while it is still in the oven. Use a long object so that your hand remains outside the oven. Alternatively, and still keeping your hand outside the oven, insert a stirrer while the container is still in the oven. (Thus, if vigorous boiling occurs, most of the boiling water will strike the inside of the oven.)

Keep your face well away from the open oven door and from the container.