When can liquid water remain above 100 C?

Well,

Typically, a substance is under 1 atmosphere of pressure, or 1 atm. More than 1 atm means there is more pressure than that which the earth's atmosphere exerts on an object near the surface of the earth.

If a liquid is given enough energy, and the atmospheric pressure remains constant, the liquid will turn into a gas. In the case of water, it will turn into water vapor at 100 °C. However, if you increase the pressure to greater than 1 atm, the water will be pushed together, keeping it liquid.