Opalchenska station entered into service on 17 September 1999 .
Serdika station situated on St Nedelya Square followed on 31 October 2000 , extending the total system length to 8.1 kilometres ( 5.0 mi ) .
Opalchenska station entered into service on 17 September 1999 and Serdika station situated on St Nedelya Square followed on 31 October 2000 , extending the total system length to 8.1 kilometres ( 5.0 mi ) .