Answer:
Heated groundwater.
Explanation:
Geysers are formed when the water unground begins to heat up geothermal energy. This then causes pressure to the water which is beginning to boil. The pressure then forces the boiling hot groundwater up until its near the very surface of the ground. The pressure then quickly leaves. Due to the sudden drop in pressure to the heated water, it causes the hot water to create a steam. The steam is then forced up the rest of the way as a geyser is formed.
Think of it as boiling water in a pot. Eventually, there is so much pressure and steam in the pot that (if you have a cover over it) it begins to rattle the lid shortly before boiling hot water manages to escape the pot while the rest evaporates to steam.
