"many a night, just at midnight, when all the world slept, it [a groan similar to that uttered by the old man] has WELLED up from my own bosom, deepening, with its dreadful echo, the terrors that distracted me. " Poe

Question

The type of comparison the author uses with the word WELL, is called a ______because it's not direct, but implied.
Anyone have any idea what type of comparison this is? I said he uses the word 'welled' in order to compare the way groan comes out of man to a fountain or even tears. They both well from a compiling of pressure until they finally burst forth into existence.