ELI5 Tunnels - how are underwater tunnels built?

How do they build underwater tunnels? How do tunnel boring machines know where they are going? What happens when they meet in the middle? Why do underground tunnels not flood when it rains - doesn't water flow to the middle and collect?

... We explain like i'm five!

Thank you to Carl for writing in with this week's question!

Also thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: ermoibliye, smodfan, 2CGMicro, chilli0102, garokev, netsecwarrior, salmon_pokebowl, wrsaunders, enrique37, bap710, kylehooks and hotfudgefries

To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter:

https://www.twitter.com/eli5ThePodcast/

or send us an e-mail:

[email protected]