to go into hiding; to operate in secret
The group of dissidents went underground.
The close call forced Gerhartsreiter to quit his $100,000-a-year job and go underground.
The patient was between forty and fifty years of age, and had been unable to go underground for several years.
They were flying over our heads and we had to go underground.
After all, it is not natural for man to go underground except for business or for the last time.
"To-day you must go underground and fetch my taxes," he said to the Prince.
In the business portion of many cities a transmission line must go underground, whether its voltage is high or low.
I have a long way to go underground and must arrive while it is still dark, so no one can mark the entrance by which I go.
Oh, why did Lisabetta go underground for ever with half the cards unread?'
When we arrived on the Canadian shore we had to go underground and array ourselves in black or yellow mackintoshes.
But I made no move to go underground to the cave to look for them.
To go into hiding; to operate in secret: the school newspaper went underground/ the persecuted go underground