An undersea volcanic formation that does not rise at least 1000 meters above the surrounding sea floor is called a seamount.

Here's the breakdown:

* Seamount: A mountain rising from the ocean floor that does not reach the surface. They are generally defined as having a relative elevation of at least 1000 meters.

* Guyot: A flat-topped seamount, often formed by wave erosion when the seamount was once near or above the surface.

So, if an undersea volcanic formation is less than 1000 meters high, it's still considered a seamount.