I agree that condensation will eventually seep in and condense into water. But ingenious planning could overcome it. Use positive ventilation, guarded from bringing in more humidity, snow, bugs, dirt, and rain system to the pipes underground. Solar power it - it will activate on the "good days". If necessary, leave access points to easily slide new optic fiber, wrapped in low friction plastic and points where the fiber needs welding. Nothing is perfect. But it will be awfully close to being permanent. Nothing is permanent in this world.. earthquakes and land movement will see to that.