SEE, WATCH, HEAR, LISTEN, FEEL, SMELL, NOTICE, OBSERVE + object + present participle

After verbs of perception (see, watch, hear, listen, feel, smell, notice, observe etc.), the present participle expresses an action that continues over a period of time, regardless of whether we perceive the whole action or only part of it:

I felt the ground shaking for about half a minute. (I felt that the ground was shaking.)
I saw the burglar climbing over the fence. (I saw it as the burglar was climbing over the fence.)
I heard my brother singing in the bathroom. (I heard my brother as he was singing in the bathroom.)

If we want to emphasise that the action is completed, we use the bare infinitive:

I felt the ground shake once. (I felt it as the ground shook once.)