Background
The science behind the books. Each term is explained the way the novel uses it. Below it, you’ll find what part of it is real. Information about the real world is current as of October 2026.
Atlantic Meridional Overturning Circulation (AMOC)
A circulation system in the Atlantic that carries heat north. Warm water flows north near the surface, cools there, grows heavy, sinks, and flows back south at depth; according to the book, the Atlantic pushes seventeen million cubic meters of warm water north this way every second. That’s why Hamburg doesn’t freeze over in February, even though it lies at the same latitude as Labrador. Lena compares it to a heating system: the damage doesn’t start when it breaks down, but as soon as it runs slower than the dikes were designed for.
Deep current (title of Book 1: TIEFSTRÖMUNG)
The lower, cold part of the overturning circulation: the water that has sunk flows “back south as a cold deep current.” According to Lena, how much of it sinks decides everything, and no satellite can see that; you have to go out and measure the depth profile. The Amazon description plays with the word: the Atlantic isn’t the only thing with a deep current.
Calibration
Calibrating means comparing a measuring instrument against a known reference value and recording the deviation. Correcting the instrument afterward is called adjustment. Lena calls the calibration records of one sensor series in the North Atlantic “generous” and explains the word: infrequent, and with a lot of leeway.
Design water level (Bemessungswasserstand)
The water level the dikes are built to withstand, in the book “the number everything hinged on.” On the Lower Elbe, it is set at a level that is statistically reached once every two hundred years; the height of the dikes, the storm surge barriers, the warning times, the bus depots, and the nursing home lists all depend on it. If the circulation weakens further, the water level in the German Bight rises faster than the global average, and storm tracks shift south. Lena spends a night calculating three scenarios: this water level would then come every twenty-seven, every fourteen, or every eight years. Two hundred years means maybe once in a person’s lifetime; twenty-seven means three times.
Mean high water (mittleres Hochwasser)
The level the tide reaches on an ordinary day. In a severe storm surge, the water rises three meters or more above it; without a dike, it would stand as high as a person on the ground floor. According to the book, Wilhelmsburg and Finkenwerder lie one meter above mean high water.
1962 storm surge
One night in 1962, 315 people died in Hamburg. Afterward the dikes were raised, with the promise “never again.” Anna says that was right: “For the Atlantic we had back then.”
Campi Flegrei
A volcano near Naples that you can’t see: no mountain, no peak, just a hollow, and the city stands in the hollow. You don’t live next to this volcano, you live inside it, and the ground has been rising and sinking for 2,000 years. From the water, Lena can see that the hills behind Pozzuoli form a semicircle: the city lies in the crater, and its rim is the wall.
Pozzuoli
A town in the Campi Flegrei, in a shallow basin between hills and water. In the middle of town stand three Roman columns with holes bored halfway up by marine mollusks: there was water here once, and then the ground rose. One district has stood empty for more than 50 years, because one March morning the ground rose so fast that 3,000 people had to leave in a single day; in 1984 half of Pozzuoli was evacuated, after the ground rose almost two meters over the course of the crisis.
Bradyseism (Bradyseismus)
The slow rising and sinking of the ground in the Campi Flegrei, accompanied by swarms of shallow earthquakes. Sofia Esposito calls the uplift the most honest and the most ambiguous of her readings: it shows for certain that something is moving, but not what.
Fumarole
A spot where steam and gas escape from the ground. Fumarole gas readings are one of Sofia’s four data series; at the Solfatara, a fumarole hisses like a leaking steam pipe, and the air smells of sulfur.
Protezione Civile
Italy’s civil protection agency, which decides in an emergency who has to leave.
Red zone
The area around the Campi Flegrei that is red on the maps. Red means: anyone who lives here has to be out before the volcano erupts; afterward it’s too late. On the street you see none of this. The red zone isn’t red; it’s lived in, with schools, shops, and laundry hanging over the alleys.
Alert level (Warnstufe)
The tiered system for the volcano, four levels in the book. Level two means: look more closely. Only at level three are hospitals and care homes relocated, and only at level four is the entire red zone evacuated. Sofia uses a smoke detector to explain why there is no simple threshold: it beeps above a certain amount of smoke, because smoke means only one thing. A volcano is not a smoke detector.
Earthquake magnitude (Bebenstärke)
Sofia translates the numbers into images: at two, the ground clears its throat, and you only feel it if you’re sitting down; at three, half the street is standing outside and the pictures hang crooked; at four, plaster falls, the schools stay closed, and nobody sleeps. That’s how Sofia explains it in the novel. Current readings are published by the Osservatorio Vesuviano.
Dam, reservoir, and maximum operating level (Talsperre, Speicher, Stauziel)
A dam, which the book also calls “the barrier” (die Sperre), holds back a river; behind it the reservoir collects, a man-made lake. On the Dorn, the dam stands for power generation and to catch floods before they reach the village. The maximum operating level is the height to which the reservoir may be filled, given in meters above the Adriatic datum. The reservoir is not a lake, the book says: it is gathered behind concrete and leans its full weight against the one wall that holds it.
Pressure tunnel and cavern (Druckstollen, Kaverne)
The pressure tunnel carries the water from the reservoir through the mountain to the powerhouse; on the Dorn it is 700 meters long. The powerhouse is a cavern, a hall in the rock almost 400 meters deep inside the mountain, with two machines that turn falling water into electricity. The water presses against the mountain, and the mountain passes the pressure on; you can feel it on the rock wall, and that is normal.
Load change (Lastwechsel)
The term for when one of the machines starts up or shuts down. The water in the mountain suddenly speeds up or slows down, and everything that carries it (pipes, tunnels, rock) feels it. This is part of the design and happens many times a day; machine one starts from a complete standstill only twice a day, at 3:40 p.m. and at 2:10 a.m.
Jolt and knock
In TIEFENSCHLAG, this is a difference you need to know. With every load change there is a short jolt from below, and that’s part of normal operation. A knock is something else: a sound deep inside the mountain that you can hear but no instrument shows. What lies behind it is the question the book revolves around.
Shelf edge (Schelfrand)
The edge of the continental shelf under the sea, in the book the Norwegian shelf. The question is whether a slope there could slide away underwater and send a wave all the way to the German coast. Every week Lena reads model notes on this, often two model runs from two institutes that don’t show the same thing at the shelf edge.
Storegga Slide
Eight thousand years ago, at Storegga on the edge of the shelf off Norway, a slope slid away beneath the sea and set the water above it in motion. A wave like that travels for hours in every direction; where the sea gets shallower, it slows down and grows taller. The book describes the North Sea as a shallow, cold basin that pushes everything coming from the north southward into the corner where the Elbe lies.