They are GINA gaskets[0], they were supposed to last 120 years[1], but it has recently been shown that they may deteriorate faster than previously expected due to being under constant compression[2][3]

[0] https://www.trelleborg.com/en/marine-and-infrastructure/medi...

[1] https://www.trelleborg.com/marine-and-infrastructure/-/media...

[2] https://www.sciencedirect.com/science/article/abs/pii/S08867...

[3] https://dailygalaxy.com/2026/03/rubber-used-in-undersea-tunn...