the cable that comes out of the ocean and goes onto the island. Sayid finds it
The underwater Dhrama Looking Glass station. Remember from the season finale of season 3 I believe, it's where Desmond and Charlie swam to to press the button thing to allow communication with the outside world, but those two women were there and then Mikhail showed up and Charlie ended up drowning after Mikhail threw a grenade at the control room Charlie was in, but before Charlie drowned he managed to tell Desmond that the freighter was not Penny's boat.... ringing a bell? The cable was to provide power to it.
