A TRACON Controller reported a departing aircraft on vectors was issued a climb clearance but descended and flew below the Minimum Vectoring Altitude.

Narrative

I was working the Radar West sector at ZZZ. At approximately XA43z Aircraft X departed ZZZ1; assigned an altitude of 5000 ft. by Tower. Upon initial contact; I assigned him an altitude of 15000 ft. which the pilot correctly read back. I also gave him a vector on course. Approximately 7 miles west of ZZZ1; there are some radio towers that go up to about 2500 ft. around which there is a 3500 ft. MVA. In accordance with our local DVA procedures; I watch Aircraft X to ensure he was above 2600 ft. prior to entering the 3500 ft. MVA. Indeed; he was at 2700 ft. prior to entering the MVA; so I continued my scan of the scope. Several seconds later; I looked back at Aircraft X to see that he was directly near the marking for the radio towers in the MVA; but had descended to 2400 ft. in the MVA. I was very surprised and told him to begin his climb; and that there was an obstruction in his vicinity; after which he once again began climbing.Aircraft X successfully climbed out of the MVA and continued on his course. For pilots to not randomly descend into an obstruction area. Additionally; in my surprise at seeing his lower altitude; I didn't issue a proper low altitude alert. In the future I will endeavor to give a proper low altitude alert when the situation dictates.