Which preposition to use with underground

in Occurrences 4%

The Nazi underground in South America.

before Occurrences 1%

And the dead of each day must be got underground before midnight of that same daythat is the hard-and- fast rule wherever the Germans are holding their ground or pressing forward.

to Occurrences 1%

It was a tracing made by a discharged convict from a dusty leather-covered book in the public library in New York, sent in by the underground to Jim.

Which preposition to use with  underground