Throwing stones in a abandoned oil shaft

Founded an abandoned and open oil shaft from old oil rig. We threw a few stones in it and the result is what you can hear and see.

The video couldn't really record how strong and deep the sound was when it got out from the shaft.

The sound waves where extremely loud especially at the end of each video - when a big and loud boom came out (like big gun firing).

After the boom, the rock seemed to have gone even deeper as high pitch noise was still coming out from the tube (like the rock was still hitting metal walls).

What was that big boom at the end of each recording??