When her three-person submersible descended more than 30,000 feet into one of the Pacific Ocean’s deepest trenches, Mengran Du wasn’t sure what they would find. What she saw, she recalled, was “unbelievable”: Dense clusters of tubeworms with tentacles tinged bloodred, jutting up like skyscrapers. Iridescent snails scaling the worms, like window washers. Bristly, white creatures wriggling between them like rush-hour commuters trying to get home for dinner.