There could be at least one supervolcano that location is unknown.

"A supervolcano is a volcano that has produced an eruption with a Volcanic Explosivity Index of 8, the highest recorded number on the index". (ScitechDaily.com/Volcanic Super-Eruptions Are Millions of Years in the Making)

Theoretically, controlling volcanoes is very easy. Decreasing the pressure in the magma chamber is enough. And if researchers can decrease the magma's temperature. They can also decrease the pressure in a magma chamber. Researchers can decrease that temperature by pumping extremely cold gas like liquid nitrogen or liquid helium into that magma chamber. 

Sometimes is introduced a way to deny the volcano eruptions is by using laser drills. The laser drill makes multiple holes in the roof of the volcano's magma chamber. Then liquid nitrogen or Bose-Einstein condensate will pump to the magma chamber. That decreases the temperature and pressure in that magma chamber. 

Supervolcanoes like Yellowstone and Toba are the most fascinating and frightening that we can wonder about. There is a certain cycle between the activity of the volcanoes. The reason for that is that the pressure in the magma chamber must rise high enough that the volcano can erupt. 

The volcanoes that erupt often are quite safe. Their eruptions are not very strong because the eruption channel is open and pressure will not rise in the magma chamber. 

And the longer the cycle of the volcano, the stronger its eruption. In the longer period break between eruptions, pressure in the magma chamber will rise higher. Supervolcanoes have the longest periods between eruptions. And that means they can sleep even millions of years. 

Image 2) 

Supervolcano eruptions can take thousands or even tens of thousands of years of making enough pressure that it can erupt. In some of the biggest possible supervolcanoes cycle, eruptions can be even millions of years. And that extremely long cycle makes those supervolcanoes even more dangerous than we expected. 

There could be at least one supervolcano that is invisible on Earth. In normal cases, volcanism is visible in infrared images. But there is the possibility that two magma chambers are overlapping. In that case, the upper chamber would belong to the regular volcano. But there could be another larger magma chamber below the smaller volcano. 

And if something happens and the upper magma chamber will empty. The weight that locks magma in the lower chamber will be released in that case. That can cause the situation that the magma of the lower magma chamber will break its roof. And the volcano that is over the magma chamber can just fall in it. 

If that thing happens at the sea that thing causes a horrifying tsunami. There is introduced to control or deny the supervolcano eruptions by drilling holes in the roof of the magma chamber. 

Engineers can make those holes by using a laser drill. And then the liquid nitrogen or Bose-Einstein condensate can be injected into the magma chamber. That thing can decrease the temperature of the magma. As I wrote at the beginning of this text. 

https://scitechdaily.com/ancient-ice-reveals-scores-of-gigantic-volcanic-eruptions-causing-tsunamis-drought-famine-and-death/

https://scitechdaily.com/the-silent-build-up-to-a-super-eruption-that-could-catastrophically-affect-global-climate/

https://scitechdaily.com/volcanic-super-eruptions-are-millions-of-years-in-the-making/

Image 1) https://scitechdaily.com/volcanic-super-eruptions-are-millions-of-years-in-the-making/

Image 2) https://scitechdaily.com/ancient-ice-reveals-scores-of-gigantic-volcanic-eruptions-causing-tsunamis-drought-famine-and-death/