An underwater suspended tunnel (6) has a shaft (10) with generally convex upper and lower outer surfaces (12, 13) meeting at longitudinally extending, transversely streamlined and opposed sides (14, 15). One or more apertures (20a, 20b) for carrying traffic extend longitudinally through the shaft (10). The shaft (10) has positive net buoyancy and is tethered at a generally uniform depth below sea level (5) by ties (32) anchored to the sea bed (4).