Is it cold in the tunnels? The temperature underground surprisingly varies very little throughout the year, as the ground provides excellent insulation! Except in extreme weather, most of the cavern system is around 10-14 degrees celcius. This can feel very cold on a hot summer's day! We suggest bringing a coat or jacket all year round. When the weather is very cold outside some parts of the tunnels cool down too, and occasionally give us some spectacular icicle displays! Whatever the time of year, you're very welcome to bring blankets and even cushions if you wish to stay cosy during your boat trip.