Creating a falling ball map

Pyth, 19 bytes

VQ.>+*\_25*d5O30*b4

Try it here!

Explanation

VQ.>+*\_25*d5O30*b4  # Q = input

VQ                   # Repeat Q times
     *\_25           # 25 underscores
          *d5        # 5 spaces
    +                # concat the underscores and spaces
  .>         O25     # rotate this string 0-29 times
                *b4  # print the seperating newlines

Ruby, 74 71 59

Thanks to Alex A. for reminding me that being basically one line means I can use the stabby lambda

f=->n{n.times{puts (('_'*25+' '*5)*2)[rand(30),30]+"\n"*5}}

Pyth, 19 bytes

VQ.>r"5 25_"9O30*4b

Try it here

Thanks to FryAmTheEggman for catching a bug.