British-born Australian resident Alexandra Harris, 27, was one of 30 people detained over a Greenpeace protest at a Russian Arctic oil rig in September.
Ms Harris describes her time in prison as feeling so alone, where she would "sit there and worry and stress about the situation".
"I didn't know where my friends were, what was going on, no-one could speak English and I couldn't communicate with anyone. I couldn't sleep, it was horrible," she told the BBC.
However, after Ms Harris discovered her cell was right next to another activist, they were able to communicate through tapping messages through the radiator pipe.
"We managed to have some conversation on the radiator pipes - one tap was A, two taps was B. Sometimes it took 23 taps to get out one letter.
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"Even though we couldn't see each other, it felt like we were together," she said.
Click on the video above to hear the full interview.
