Although not really a hobby tunneler, one of my favorite people on this list is William John Cavendish-Scott-Bentinck, the reclusive 5th Duke of Portland who built an extensive series of tunnels under his estate. His life is fascinating. Recommended reading:

https://www.atlasobscura.com/articles/the-duke-who-loved-tun...

http://www.nottshistory.org.uk/portland1907/portland7.htm