So i've found a good sub to use, went ahead to test it. I've chosen a non mission path and started the mission. I've encountered either two mantises or tiger threshers, when suddenly my screen starts shaking and my submarine is pulled down ( no idea if it's a glitch.) . The sub hit the bottom and i could see some sort of smoke coming from the ground. the water had a redish color. Can anybody explain to me what happened?

You're passing over a hydrothermal vent. The hot water is less dense, causing the submarine to lose depth.

Thanks.

Plus hydrothermal vents deal damage to cremates outside submarine.