Tunnels

• Wikipedia on large tunnel-boring machines - [1]
• There is huge red tape for building tunnels, and costs are $100M-$1B per mile [2]. The Boring Company was supposed to do $10M/mile
• Boring machines - called moles - cost a million. They work like this [3]
• In soft rock Youtube - digging for gold [4]
• Rock bolts are used to stabilize a tunnel - [5]