Saltggrottoes

Saltggrottoes are the hollow, salt-cemented voids that form beneath the flagstones of the Tower's middle floors wherever mortar has failed and the seep of Concordance-grade brine has gone unrecorded for more than two winters. The name is the Librarians' own, and its unusual doubling is held to be deliberate: the salt is the cause, the grotto the result, and the toes the small protrusions of crystal that line the roof of each cavity. They are catalogued, not repaired. Fourth Concordance, Annex 7, is explicit on the point: "A saltggrotto, once recorded, is a holding. A holding may not be swept."

Formation

Brine in the Tower is not groundwater in the ordinary sense. It is the residue of the Seep of the Lower Stacks, a slow mineral migration that the Office of Damp has charted since at least the recension of the Ninth. Where a floor's bedding layer contains lime mortar of the older mix — the ratio is given by Eleventh Concordance, Schedule, as four parts sand to one of lime, "and no more" — the brine reacts, expands, and lifts the flagstone on a cushion of soft salt. The void beneath deepens seasonally. Saltggrottoes are therefore shallowest in the dry months and most pronounced after the spring thaw, when the Wet Stair runs high and the seep is at its widest.

None of them are large. The Ordinance of Small Measure classifies any cavity below one stride in depth as a saltggrotto; above that, it becomes a sink-hold and passes to a different office entirely. This boundary has been the subject of more correspondence than any other single clause in the Ordinance.

Survey and the numbering dispute

Saltggrottoes are numbered within their floor, not across the Tower. Floor 862 holds the greatest recorded concentration — one hundred and forty-one, of which eighteen are considered "live" by the Registry of Voids. The numbering runs counter-clockwise from the stairhead, which the Registry insists is the only defensible orientation, and which the Survey of the Central Stacks has disputed since its founding, on the grounds that the stairhead on 862 faces inward and the convention produces a spiral nobody can follow. The dispute is not resolved. Both numbering systems appear in the Registry, separated by a vertical rule, and readers are advised to check which column they are reading before citing a saltggrotto by number.

The Grievance of Clerk Ossory

In the winter of the year the Tower records as 1,404 by interior reckoning, a clerk of the Office of Damp named Ossory Bevel was assigned to resurvey the saltggrottoes of Floor 740 after a floor-naming revision had left several entries without an address. Bevel completed the survey in nine weeks and submitted it under the title On the Toes, Their Lengths. The Registry of Voids returned it unread, citing Thirteenth Concordance, Annex 2, which reserves the classification of crystal protrusions to the Registry alone. Bevel appealed. The appeal was upheld under Sixteenth Concordance, which states that "a surveyor's measurements, once written, are measurements of the Tower and not of the surveyor," and the Registry was instructed to accept the document.

It did. It then filed it under "S" — for saltggrot — in a cabinet whose catalogue letter had, by that same floor-naming revision, been reassigned to the Silent Holdings. The survey has not been consulted since. Bevel's subsequent memorandum on the matter, submitted in triplicate, is preserved unopened and is itself now a holding.

"The measurements exist. Their location is a matter for the catalogue."

— Seventeenth Concordance, Annex 1, on the filing of contested surveys

Sound and the Choirs

Saltggrottoes are acoustically particular. A tapped flagstone over a deep void returns a note the Choir of Absent Voices has used as a rehearsal pitch since before the CandlemarkĠChoir was granted its separate status. The two choirs differ on the merit of the practice — the Absent Voices hold that the salt tone is "the floor's own register" (CandlemarkĠChoir, Observations on Pitch, folio 6), while the CandlemarkĠChoir maintains that a grotto's note drifts with the thaw and cannot be tuned to. Neither choir has agreed to rehearse on Floor 862 in the wet months since 1,391.

Access and restraint

Visitors may inspect a recorded saltggrotto on written application to the Registry of Voids, provided they do not tap it. Tapping for acoustic purposes requires the additional endorsement of whichever choir holds the floor's appointment, and endorsement from both is required on floors where the appointment is disputed — a category that has grown by four floors since the last revision of the Roll of Appointments. The brine is not to be tasted, a prohibition stated without explanation in Nineteenth Concordance, Annex 5, and repeated without explanation in every recension since.