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Cesium-137 or Caesium-137, represented as Cs-137 or 137Cs, is a radioactive isotope of cesium. It is a component of the dark matter - a concentrated form of the God particle - which was produced by the 1986 incident at the Winden Nuclear Power Plant.[1] The terms cesium, dark matter and God particle were sometimes used interchangeably, when time travelers discussed the fuel that powered their machines.


In real life, Cesium-137 has a number of commercial uses, including roles in radiation therapy, various types of radiation and moisture gauges, and radiometric dating.

In the parallel worlds of Adam and Eva, the substance is a kind of fuel that allows for the travel through time and space.



September 22[]

The alternate Martha Nielsen travels to 1888 after rescuing a teenaged Jonas Kahnwald from the apocalypse in 2020 on the orders of Adam, Jonas's older self.[2][3] There she meets the middle-aged self of Jonas, who got stuck there along with the teenaged Bartosz Tiedemann, Magnus Nielsen, and Franziska Doppler after escaping the apocalypse.[4] He is immediately distrustful of her, because she is not the Martha that he knew in his youth, and he never traveled to her world as he did in an overlapping reality. In order to gain his trust, Martha offers up a small sphere of Cs-137, which she claims is her last; she will not be able to return. Jonas accepts it but has no luck doing anything with it. While the others are distracted with Jonas's burgeoning time machine, Martha flees using more cesium she was hiding.[5]


June 21[]

An incident within the volume control system occurs at the Winden Nuclear Power Plant in the night. As a result of the meltdown, a vast quantity of radioactive waste is produced.[6] It is called a "cesium residue",[7] but more specifically it is Cs-137, an isotope (variant) of the base element cesium.[8] The plant's director, Bernd Doppler, has the waste stored in tens of yellow barrels and hidden in the "interim facility" of the plant's section of the caves.[6]

November 5[]

When Bernd retires as director of the plant and transfers power to Claudia Tiedemann, he reveals to her the barrels containing the Cs-137.[9]

November 12[]

A middle-aged Jonas Kahnwald meets with the clockmaker H.G. Tannhaus in his shop. Tannhaus has at last finished his portable time machine, after Jonas brought him an older version of it and he could see its finished state. Jonas draws a vial of Cs-137, which he acquired in the year 2019, explaining to Tannhaus what it is and where it is from as he inserts it into the newly finished machine. When he makes to leave, Tannhaus inquires what the future is like. Jonas responds cryptically that hopefully by tomorrow, it'll already be different than it is.

That night, Jonas brings the portable time machine with the Cs-137 into the passage in the caves. He wants to destroy the wormhole within it once and for all. When he activates the machine, however, he only ends up closing it temporarily.[8][10] In reality, Jonas allows for the continued existence of the wormhole because he brings the Cs-137 into the passage before the isotope has fully decayed. Cesium residue is left behind, allowing the passage to be reopened at a future point in time (also by using the Cs-137, which remains behind again to fuel the passage and thus perpetuate the loop).[7]


June 22[]

An older version of Claudia visits her in her office in the power plant. She reveals to herself the existence of time travel and then passes along a map among a bundle of papers. The two Claudias go down to the yellow barrels in the caves, where the older Claudia demonstrates the usage of her portable time machine. She fuels it using a vial of Cs-137. Upon the younger Claudia's question of what that is, the older one says that it is both "mankind's greatest discovery" and also "its doom". The older Claudia travels away.

That night, the younger Claudia digs up the time machine in her backyard using the map her older self gave her. Inside the machine is the rest of the Cs-137 after the older Claudia used it once to go back to 1954 and bury it.[11]

June 23[]

Noah inserts a vial of Cs-137 to fuel the upgraded time travel chair in the bunker at the Doppler cabin. He sends the child Helge Doppler back to his native 1954.[12]

June 25[]

Claudia gives the vial of Cs-137 to a scientist at the plant to test it and determine what it is precisely. She orders him not to speak of it to anyone.[13]

June 26[]

Claudia voice records a description of the data acquired from the tests. The particle regularly decays into two protons, at a mass of 125 gigaelectronvolts. In at least one case, however, it decayed into four muons. She is then interrupted by her secretary Jasmin Trewen, who reminds her of the date.[14]

June 27[]

A teenaged Jonas Kahnwald and Claudia go to the caves where the yellow barrels are hidden. Jonas has just spent the past year training with Claudia's older self and has learned a lot. Although he knows his older self failed to close the passage the year before, he still believes it is possible to change things; one need only change a grain of sand, and that can change the whole world. He opens one of the barrels and takes some of the Cs-137 into a vial.

They go to the passage in the caves. In the tunnel, Jonas activates the portable time machine. Spinning tendrils of shimmering particles emerge from the machine. He says they are connecting past and future. After they reopen the passage, they use it to travel to 2020, where the apocalypse is drawing close.[4]

Again, unbeknownst to Jonas, his bringing the cesium into the passage before it has fully decayed does nothing but fuel its existence.[7]


In 2019, a year before the plant is set to decommission,[15] it still holds the yellow barrels of Cs-137 in its possession in the caves.[16]

November 5[]

Leary of the police investigation into the disappearances of Erik Obendorf and Mikkel Nielsen, Aleksander Tiedemann, the director of the plant, orders Erik's father Jürgen, a driver at the plant, to transport the barrels to another location. Aleksander oversees this transport later that night.[16]

November 11[]

Jonas Kahnwald breaks into the truck holding the yellow barrels. He opens one of them to take a some of the Cs-137. He heads to see Tannhaus in 1986, who has finished building his portable time machine.[6]


June 20[]

Aleksander has the yellow barrels of Cs-137 buried underneath concrete within the power plant. After the building is demolished, this room will become part of the green field.[10]

June 27[]

On the day the plant is meant to decommission, the existence of the Cs-137 is at last uncovered by the police. Inspector Clausen, the head of the new federal task force assigned to the missing persons case, inspects the nuclear power plant after arresting its director. He comes across the old cooling pool, which he wants searched. Inside, he has the concrete drilled up to reveal the yellow barrels. Despite the protests of Charlotte Doppler, Winden's chief of police, he has one of the barrels opened. Upon exposure to the air, the Cs-137 rises up out of the barrel and begins to shift into a giant black orb, glowing a bluish hue. After stabilizing, it shrinks down into nothing, disappearing into thin air. In its place, a wormhole opens, connecting the years 2020 and 2053. Charlotte sees an older self of her daughter, Elisabeth, on the other side, in the ruined power plant of the future. When their fingers touch through the wormhole, a black dome descends over the power plant and expands to engulf the world.[4]

September 25[]

After the apocalypse, Claudia is visited by an alternate version of herself. This alternate self explains to her the nature of Cs-137 and how it is connected to the existence of the passage in the caves.[7]


Elemental cesium, assigned the atomic number 55, is a soft, shiny, silvery-golden alkali metal. As such, it has properties similar to sodium and potassium, including being highly reactive with water; as such, it must be stored in oil. It has a low melting point of only 28.5°C (83.3°F).

Cesium features prominently in two arenas at the center of Dark: timekeeping and atomic fission. The very definition of the second is based on the transition of the cesium-133 atom, and world time and frequency standards are set by atomic clocks which use cesium-133.

Cesium-137, specifically, is one of the two main "medium-lived" byproducts of uranium 235 and plutonium-239 fission (along with strontium-90). It can be highly dangerous because it is produced in large quantities, has a half-life of about 30.17 years, and reacts readily with water, making it easy to absorb into soft tissue.


  1. Dark Season 3 Episode 5: "Life and Death"
  2. Dark Season 3 Episode 1: "Deja-vu"
  3. Dark Season 3 Episode 3: "Adam and Eva"
  4. 4.0 4.1 4.2 Dark Season 2 Episode 8: "Endings and Beginnings"
  5. Dark Season 3 Episode 2: "The Survivors"
  6. 6.0 6.1 6.2 Dark Season 1 Episode 9: "Everything Is Now"
  7. 7.0 7.1 7.2 7.3 Dark Season 3 Episode 5: "Life and Death"
  8. 8.0 8.1 Dark Season 1 Episode 10: "Alpha and Omega"
  9. Dark Season 1 Episode 3: "Past and Present"
  10. 10.0 10.1 Dark Season 2 Episode 1: "Beginnings and Endings"
  11. Dark Season 2 Episode 2: "Dark Matter"
  12. Dark Season 2 Episode 3: "Ghosts"
  13. Dark Season 2 Episode 5: "Lost and Found"
  14. Dark Season 2 Episode 7: "The White Devil"
  15. Dark Season 1 Episode 1: "Secrets"
  16. 16.0 16.1 Dark Season 1 Episode 2: "Lies"