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I'm not completely sure this is on topic. I apologize if it's not.

I have a 24-word seed stored (etched on metal) in my house. I continue to worry (just a little) that the metal might either fail to survive a fire or end up buried under so much rubble that it will be outside my control. So I'm using a backup storage system, and my question is whether it is insecure in ways I haven't thought of.

Namely: I have a Trezor (with the same seed and with a PIN number that I am confident is quite strong and secure) buried a few inches underground in a waterproof box, on my own property but quite far from the house (and so presumably safe from house fires). I can imagine that somebody might stumble on this somehow one way or another, but in view of the strong and secure PIN number I am not concerned about this.

I ordinarily walk by the burial site a few times a week, so will notice if it's been disturbed, in which case my plan is to bury a different Trezor, possibly in a different location.

It's also possible that the waterproof box is not as waterproof as I think it is, but I expect that the likelihood of losing both my metal seed backup to a fire and my Trezor to the elements is acceptably small.

I could, of course, put that Trezor in a safe deposit box instead of burying it. But I am imagining a scenario where my seed phrase is gone; I'm not sure whether it's been destroyed or found by a firefighter, and I want to move my coins in a hurry, possibly on a weekend and surely without having to spend time at the bank.

Is this plan less safe than I think it is? And if so, can you suggest an alternative that addresses the same issues? And finally, am I worrying too much about extremely unlikely events and not enough about others that are more likely but that I've failed to consider?

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Generally it is advised that you don't bury hardware wallets for long periods of time as they need to semi-regularly have their firmware updated to address vulnerabilities that get discovered over time. Instead what you should generally bury is the mnemonic seed on metal (encrypted with a passphrase or "seed extension" if it is possible an individual could find it or see you accessing it).

To state the obvious if anyone sees your unencrypted mnemonic seed they can access and move your funds.

However, in your case (everyone's circumstances are different) you want to have the option of moving the funds quickly from the burial site using a hardware wallet hence you are burying an actual hardware wallet. This seems ok to me though you should update the firmware on the hardware wallet in the case that you were ever to move funds using it. The hardware wallet may not work if it has been left for many years or decades whereas the mnemonic seed is designed to persist over time.

It seems like you have given a lot of thought to the risk of your house burning down. The probability of this happening (depending on where you are located and how common wildfires are) is low. I would encourage you to also consider other risks such as an individual (e.g. burglar) accessing the mnemonic seed on the metal in your house and whether the mnemonic should be encrypted with a passphrase or "seed extension". The upside to this is that it protects against a burglar but the downside to this is that if you forget or lose your passphrase you won't be able to access your funds.

You could also consider having your funds stored in a multisig address (e.g. 2-of-3 or 3-of-5) rather than a single signature address. This protects you against the 5 dollar wrench attack as there is no one location from where there are enough keys to move your funds but it does require more planning and technical competence.

Security is never perfect. You can only seek to improve your setup so that you can easily access your funds whilst making it as hard as possible for another individual to access your funds. It seems like you have given sufficient thought to this and your setup is satisfactory (perfection is unattainable).

This What Bitcoin Did episode with Shinobi on Bitcoin wallets and this Q&A with Andreas Antonopoulos has additional guidance and may be of interest.

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  • I had not thought about the firmware issue; thanks for this. Re burglars, I believe my seed to be well enough hidden that fire is the greater concern.
    – WillO
    Jul 4 at 23:33

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