An underground tunnel has two openings, with one opening a few meters higher than the other. If air moves past the higher opening at a greater speed than it moves past the lower opening, what happens inside the tunnel?

There would be a pressure drop in the direction of the higher opening. This will force air to move in from the lower opening and force it to leave through the higher opening. This will create a convectional movement of air, cooling and ventilating the tunnel.

Explanation:

This is in accordance with bernoulli's law of fluid flow which states that the pressure exerted by a moving fluid is lesser than it would exert if it were at rest.