Verified answer

You need to catch the flag of a buddy within "picto-chat" distance. Otherwise, you'll be down there alone...

e.z i do all of it the time 2 mah lil' sis you bypass underground witha frind and u discover there secret base enter it, and bypass 2 the computing device and push A and make a run decrease back 2 your base b4 your frind steals it decrease back!!! even as u get 2 your secret base push A on your computing device!!!!

The underground doesn't have Wi-Fi, you will need to find some one with in real life to do it.