Answer:

Explanation:
The Hi line of the Balmer series is emitted in the transition from n = 3 to n = 2 i.e.
and 
The wavelength of Hi line of the Balmer series is given by :




So, the wavelength for this line is 550 nm. Hence, this is the required solution.
Answer:
It hit Earth like a really long time ago, so many rocks and soil should have been piled on top of the crater because of wind, rain, etc.
Also, it is really deep and really old so when it is old, it gets less visible to the naked eye.