How many underwater volcanoes exist and are any actively erupting right now?

The answer

Scientists estimate there are over one million seamounts (underwater volcanic mountains) on the ocean floor, with roughly 100,000 rising more than 1,000 meters above the seabed. About 75% of all volcanic activity on Earth occurs underwater along mid-ocean ridges spanning 65,000 kilometers. The Hunga Tonga-Hunga Ha'apai eruption in January 2022 was one of the most powerful volcanic events in recorded history, sending an ash plume 58 kilometers high and generating tsunamis across the Pacific. Submarine eruptions create new islands, such as Surtsey off Iceland in 1963. The Axial Seamount off Oregon erupts roughly every decade and is the most closely monitored underwater volcano.