Read the passage and answer the question. Passage: The Sundarbans, a vast mangrove forest in the delta of the Ganges, is home to the Royal Bengal Tiger. These tigers have adapted to swimming in the salty waters and hunting in the dense swamps. Rising sea levels, however, now threaten their habitat. Conservationists warn that without urgent action, large parts of the forest could disappear within decades. Question: According to the passage, what threatens the tigers' habitat?