Station III
"Machinery hums behind high steel walls. Up the hill, there are visible outlines of warehouses and a building with a spire. But the lamps are low where they burn at all, and your ship the only one in harbour."[1]
Station III is a top-secret facility located on an island in the Unterzee.
Nothing to See Here
"On the very edge of London's influence, curious matters are conducted."[2]
Station III is a mysterious location occupied by whirring machinery, sprawling warehouses, and a tall, spired building, all surrounded by a high metallic fence.[1] The station's gate is notably cold to the touch and is nigh-indestructible, repelling even the finest of chisels and mightiest of crowbars.[3]
The only evidence of outside visitation at the port's dock is a strange burner for incense, a log book detailing an enthralling song, worn ironmonger's tools, and broken medicine bottles covered in clumps of silvery metal, with a torn label reading "SOOTHE & COO...".[4]
Most curiously, any reports of this location's activities are relentlessly censored by the Admiralty and the Ministry of Public Decency.[5]
Heartmetal
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"What use is heartmetal to you? But then, what use is a Long-Box? (Perhaps you imagine the whisper at shoulder - 'What use, indeed, is a heart?'. When you turn, there's only the ragged flirting of the wind.)"[6]

Station III is the manufacturing hub of a strange, silvery substance called heartmetal.[7] This material is extremely strong, icy to the touch,[8] and is considered dubiously legal to trade or possess, assuming the law even acknowledges it exists at all.[6] Its durability means it has a variety of practical applications,[9] but the means of producing heartmetal are kept strictly secret, for a very good reason.[10]
Heartmetal production is primarily managed by a company called Soothe and Cooper,[11] named after its founders who have a workshop of their own at Station III.[12]
Heartmetal Extraction
"On first sight, it looks like a tomb-colonist coffin. On closer examination, it carries stencilling reading SOOTHE & COOPER and a handwritten tag: 'DELIVERY TO DEPOT (A), STATION III'."[11] "Excellent, I'm going to need a scalpel and pliers."[13]

To produce heartmetal, Station III primarily relies on a steady supply of corpses stuffed inside Soothe & Cooper Long-Boxes,[11] which have been delivered to the Station for generations. These specialized coffins are brought in mainly from the Surface,[14] though they can also be found at other locations across the Unterzee, such as the Shepherd Isles,[15] Venderbight,[16] or certain shops in Khan's Heart.[17]
The corpses inside Soothe and Cooper Long-Boxes are permanently dead,[18] and are processed by the workers of Station III, known as acolytes.[19] These mysterious morticians use scalpels to carefully dissect the bodies they receive and extract the metallic deposits within. The metal spheres are layered like gobstoppers, so the acolytes crack them open to obtain their valuable, heavy cores which resemble black pellets. These pellets are then placed in jars marked with the Correspondence,[20] with the useful ones marked for export.[21]
Secrets and Tonic
"SUFFERING HEART-ACHE? A SOOTHE & COOPER tonic for the dysphoric, despondent and depressed! Equally good for catarrh. CONVENIENT, SCIENTIFIC, INFALLIBLE!"[22]

All of the deceased brought to Station III habitually took Soothe and Cooper's Heartsease Tonic during their lives. This medicine is advertised as a cure for sadness,[22] and for keeping secrets unspoken.[10] It is a strange liquid with a silvery color, and when spilled, it hardens like ice into a cold,[8] brittle, and very sharp substance.[23] This tonic is manufactured by Soothe in his workshop, where he melts leftover heartmetal shavings in a cauldron for later bottling.[12]
Soothe and Cooper's Heartease Tonic is usually consumed with wine or laudanum, but can be directly imbibed as well. Once in the body, it produces a chemical reaction with the drinker's soul, covering the source of a person's woes and slowly hardening and accumulating over time. This process creates metallic spheres that may form anywhere within the body,[10][24][25] though the most common place of the metal's accumulation is right over the heart, giving the substance its name.[20] Unfortunately, consumers of Heartease Tonic often grow dependent on it, allowing the metal deposits to grow and grow until they risk damaging internal organs.[10][20] Rubbery Men can fall under the effects of Heartease Tonic, though unlike humans, their secrets manifest in the form of black amber within their bodies.[26]

The specific location of heartmetal buildup can vary depending on the nature of a person's secret. For example, if a person listens to secrets regularly and does not speak them, the deposits may accumulate in their ears and tongue,[27] while if an academic keeps a discovery hidden, the deposit may develop within their brain.[24] The black pellets found within the cores of heartmetal deposits contain the tonic-imbiber's deepest secrets, and this pellet may vary dramatically in size depending on the intensity of the secrets.[28] Exposure to said pellet can make a person relive the memories and feelings associated with that secret, which can be extremely disorienting,[25] so the acolytes of Station III consider it of utmost importance to sing while they perform their duties; not doing so can result in a free reservation at the Royal Bethlehem Hotel.[29]
Exports and Manufacture
"They were in love with a writer who was dead before they were born, and kept it a secret from embarrassment. The Masters will like that."[30]

The Masters of the Bazaar are very interested in heartmetal pellets related to love and romance,[30] and they're the ones responsible for the creation and arrangement of Station III.[14] Any heartmetal bits that aren't useful to them, however, are thrown to the Station's dump,[31] which is sometimes visited by marauders and thieves.[32]
The Echo Bazaar has a hand in the business of Station III,[33] but it also seems to have a heartmetal operation of its own that's independent from the island; many Soothe and Cooper Long-Boxes can be found deep within the Bazaar's bowels, and can be sold legally to Crawcase Cryptics for a good sum.[34]
The Heartmetal Smith
"Nothing like it for weaponry. Does more damage than any ordinary steel, and a little bit of the shrapnel will give you trouble to the end of your days."[9]

The Heartmetal Smith is a mysterious individual who is capable of working with discarded heartmetal to forge products for export.[35][36] He often creates products suitable for maritime combat, such as ammunition, hull plates, and harpoons, as according to him, heartmetal weapons are more powerful than their steel counterparts, and their shrapnel can cause chronic ailments that last for a lifetime. Unfortunately, the hardness of heartmetal tends to wear out his tools quickly.[9]
By his own account, the Smith came to Station III by swimming,[37] and now he toils without complaint in the solitude of his workshop, with a heartmetal locket and chain always fastened around his neck.[38][39] His ultimate goal is to eventually forge a suit made of heartmetal, so that he may walk where he came from without experiencing the grief of others.[40]