To strengthen the defence further, by 1881, mines were laid in the waters between the two forts and the eastern entrance of the harbour. The mines were tethered to the seabed but floated dangerously under the surface of the water unseen to any enemy marine vessels. To enable safe passage of ships, a 'friendly safe channel' was created through the minefields. The work involved in the laying of minefields under the narrow channel (just 240 m apart) had actually led to rumours that tunnels running under the channels were constructed to link the two forts. There are, however, no archival records to prove that these tunnels were ever constructed.