1. subway systems or underground railway networks
Usage: plural form
2. secret organizations or resistance movements operating against established authority
Usage: plural form

Examples

• Many European cities have extensive undergrounds that connect major districts.
• The London and Paris undergrounds are among the world’s oldest subway systems.
• During wartime, various undergrounds helped people escape occupied territories.
• The resistance undergrounds coordinated their efforts across multiple countries.
• Modern undergrounds use electronic ticketing systems for passenger convenience.
• Historical undergrounds often used coded messages to communicate safely.