1. existing beneath or below something else
2. forming the basis or foundation of something; fundamental
3. not immediately obvious but present as a cause or basis

Examples

• The underlying rock formation was exposed after the landslide.
• We need to address the underlying causes of poverty.
• There was an underlying tension in their conversation.
• The underlying assumption proved to be incorrect.
• His underlying health condition made him vulnerable to infection.
• The underlying message of the story was about forgiveness.
• Scientists studied the underlying structure of the material.