Complex sentence: The Oslofjordtunnel is one of the longest underwater tunnels of its kind in Northern Europe; it is 7.2 km long and reaches a depth of 134 metres below sea level, with a maximum gradient of 7%.
Split into simpler sentences: The Oslofjordtunnel is one of the longest underwater tunnel of its kind in Northern Europe. The tunnel is 7.2 km long and reaches a depth of 134 metres below sea level with a largest gradient of 7%.