Generally, we know water has begun to boil when bubbles begin forming on the surface. To most, this is proof the water has boiled, but those bubbles doesn't always have to exist. Researchers at Northwestern University went ahead and boiled some bubble-free water.
This isn't the first time the effect has been achieved, but it is a new way. It's based on the Leidenfrost effect, which you can easily observe when you drop water into an extremely hot frying pan. The water skitters about in small droplets, never forming into a puddle.
Researchers at Northwestern covered steel balls with a nanoparticle-based coating giving them a rough texture, heated them to 400 degrees Celsius and dunked them in already-hot water.
The water didn't bubble against the balls. Instead, hot vapor filled the grooves of the rough coating, and the water around it, though boiling, was completely undisturbed.
This could be helpful in chemical and nuclear plants, where liquid water touching hot metal could cause explosions.