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Comment by prometheus76 | Hacker News Reader

I don't anything about "bradyseism", but I am having a hard time wrapping my mind around it. From the article: "Due to bradyseism, a geological phenomenon in which the ground sinks or rises due to pressure changes under the earth’s surface, the house ultimately fell into the Gulf of Pozzuoli."

How would this happen in such a uniform way, such that the precise configuration (and the flatness!) of the floor is preserved? Wouldn't this have been an uneven process that would have broken the floor apart?

It seems more plausible that this floor being underwater now is the result of a rise in sea levels. Can a geologist explain how a floor remains flat and a mosaic remains largely intact while sinking into the ocean?

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If you visit Mycenae in Greece, there are notes for visitors there explaining why the Greeks built such a complex city in that location, which seems to have no particular importance today ... looking out from the city towards the sea is a large flat plain. Apparently, when Mycenae was built, almost all of that plain was underwater, and the city sat on the edge of the ocean. Same process, only in this case, a slow, steady uplift out of the sea rather than a descent into it.

https://www.youtube.com/watch?v=xIpsQb5RtT4

Different phenomenon, but there's a bunch of hills in wales that have "island" names because they used to be islands in a bay. All the land in the bay was reclaimed in the early 19th century.

And the famous city of Ephesus moved several times because the port kept silting:

https://www.youtube.com/watch?v=NrXd7UCuxsk, before it eventually became just completely landlocked and abandoned.

https://en.m.wikipedia.org/wiki/Bradyseism

Seems like a very slow process that happens on a wide scale and deep underground.

The key bit in that article is this:

> caused by the filling or emptying of an underground magma chamber

In this case, the magma chamber underlies the entire area of the Phlegraean Fields. As more or less magma enters the chamber, the ground in the area slowly moves up or down.

I think you could watch some of the youtube videos the Professor Shawn Willsey has posted about the Icelandic volcanic activity over the last 6 months. As well as lots of cool stuff about the actual volcanoes and lava floes there is a lot of information about the magma chamber underneath the peninsula that is filling up and raising the land in a very measurable way. That's over a time scale of months. I think you'll see it's very plausible that over decades and centuries the level of land can change dramatically with respect to a the local 'base' level.

I understand that land can change levels over time due to magma. What is harder for me to understand is land doing so in a uniform way that maintains a flat floor and leaving intricate mosaic patterns intact. Especially when you take into account the earthquakes that would also be a part of a magma shift like that.

It's obviously possible, but hard to understand how the tensions would be spread evenly as the level raised up and down.

I know nothing about this specifically. To me, it makes sense if you think about just how big these fields are and how slow of a process it is. Changes that take place slowly over a large area are more likely to cause shifts that seem relatively flat at the scale of a house. Another way to say it: at the geologic scale, a tile mosaic may as well be a point on a plane. Unless there is a fault line at that exact position, I can see how the area would move up and down relatively uniformly.

What I’d like to know is how long of a process was it really to lose a city into the bay. It would have happened while people were around, so is there some kind of historical record?

Most likely raise of sea level? Anecdotally, I am originally from the south of Italy and at different stages both my father and grandfather mentioned to me that the seashore was much further away than the current one with the one my father recalled midway between the one my grand father remembered and the one we have now.

> It seems more plausible that this floor being underwater now is the result of a rise in sea levels

Ah yes, the "it must be climate change" response. We've been "boogey booed" into thinking that is the first and most likely cause any time we see an environment change... to the point where some people are unwilling or unable to accept the world is a constantly changing environment, even without human intervention.

Is it really hard to believe if a significant sized piece of land sinks, that things on top might just sink in-place too? You can look at concrete pads and foundations to see this in practice every day. We can also see more sudden and dramatic examples with sinkholes (caused by underground erosion of limestone and similar).

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