Related Clues:

• Become more profound
• Dig further
• Dredge, perhaps
• Drill farther
• Compound
• Grow more intense
• Make stronger
• Refines, in a way
• Thicken, as a mystery
• Extend farther down
• Drill further
• Get more complicated
• Make more navigable, as a channel
• Increase in depth
• The way to dredge the river enclosure?