A seamount if basically an underwater mountain. If it is tall enough to break the surface of the ocean we call it an island.
Tam got me started on this one. She's talking about geology and islands, which leads to the Great Meteor Seamount, which make me wonder how it got it's name. Did a meteor fall into sea nearby? Did someone see a meteor about the same time as they discovered this lump in the ocean floor? No, it was named after the ship that discovered it in the early 20th Century: the Meteor. More poking around leads to this really cool sonar recording of the Nashville Seamount, which is about 1700 miles to the WNW.
Note that the scales along both the X and Y axis are not in distance as you might expect, but in time. The horizontal axis is marked in hours of the ship's travel time. Each division of the horizontal scale is about 12 miles. The vertical axis is marked in the number of seconds it takes sound to make a round trip to that depth.
The speed of sound in seawater is about 1560 m/s (meters per second) which is not quite one mile per second. That is about four times as fast as it is in air. Since each
division of the vertical scale represents one second, and we are talking about round trip time, this means the distance between index marks on the vertical scale is about one-half mile. The
vertical scale is expanded relative to the horizontal scale by a factor
of about 24 to one.
The difference between the top of the seamount and the base is a little over 4 intervals. At one-half mile per interval means this seamount is sticking up something more than two miles (or 10,000 feet) from the bottom of the ocean floor. I guesstimate the length of the rise is about one hour, or 20 kilometers, or about 12 miles. Which means the average slope of this underwater mountain is about one foot of rise for every six feet of travel.