The pressure at the bottom of a under water tunnel is higher than at the entrance that's why your ears will pop some times. But the pressure differential between the ai inside and the water outside is much greater.
Say the tunnel bottom is 45 feet under water pressure outside of tunnel underwater is 2.5 atmospheres, pressure inside the tunnel is 1 atmosphere plus 45 /10,000 (effectively very little air above that) or 1.005 atmosphere
Say the tunnel bottom is 45 feet under water pressure outside of tunnel underwater is 2.5 atmospheres, pressure inside the tunnel is 1 atmosphere plus 45 /10,000 (effectively very little air above that) or 1.005 atmosphere