Related Clues:

• & 3 & 5 Plunging into lower parts of the ocean
• Heartfelt couple of notes associated with old record
• Pressure at bottom of river, a long way down
• Difficult to comprehend river patrol's leader
• River Po's opening leading to sea
• Like one end of the pool
• Way to go on the gridiron?
• Bottomless, seemingly
• Sunk low in river, first of piles
• Worthy of pondering
• Like abysses
• Bass, e.g
• Far from superficial
• Intensely felt
• 'Whoa, that's a really philosophical idea...'