the green boxes we have to found in the Underground, they are found in a random way or or it has a correct place to click to find the box?

Skitters gives you a green box as a random event, "COME SEE SKITTERS FOR SPECIAL SURPRISE." i don't think they're found in the underground at all :) (You have to be a zombie to get the event)

The underground only gives the random items. The boxes are Skitters' pastry boxes (RE only - stage 6 zombies and up)

the what i gain going in the Underground?