That's by the way the reason there are so many tunnels in Stockholm but very few in Göteborg. It is very easy (relatively speaking) to dig tunnels through grani

That's by the way the reason there are so many tunnels in Stockholm but very few in Göteborg. It is very easy (relatively speaking) to dig tunnels through granite but very hard through muddy sediment. Some tunnels are still being dug in Göteborg but they are much more expensive than in Stockholm. Nowadays, one of the main problems of digging in Stockholm is that there are already so much stuff dug out that it is hard to find room for new tunnels!