It always fascinates me when you have a combination of the close-up and the angle of the light such that you can see that the facing or skin or siding on these machines is not perfectly flat or smooth, that, indeed, there is a ripple effect, that the skin is not perfectly tight. Counter-intuitive, always amazing. Gorgeous shot, like from the opening promo scene in Homeland when Marine One flies directly overhead.

What is the open door bottom center?

the open door is for loading and unloading the hook for external loads, the loadmaster can guide the pilot to pickup a container, another helo, or a vehicle this way

I have certain shots of Boeing Commercial Airliners I'll try and find, especially in the lower fuselage aft of the wing that reveal dimples, that show off this same sort of characteristic in the skin, testament of many flight cycles I presume.