In planning the Southeastern Architectural Archive's Medieval Louisiana exhibit, we came across a number of relevant historic news items. This mention in The Christian Observer dated 19 March 1840 piqued our curiosity:
Dungeons of the Inquisition in New Orleans
"A curious discovery has been made by some workmen employed in erecting houses on the site of the old Calaboose. That ancient building, which dates far back into Spanish times, was recently pulled down, and the ground on which it stood sold out to private individuals. The purchasers immediately commenced improvements upon the property, being valuable, from its location in the centre of the city. In the course of operations to this effect, it was found necessary to dig several feet under the surface, to lay a substratum for the walls of the houses about to be built. The labourers, in excavating at a particular spot, discovered that their progress was retarded by some hard substance, which resisted any impression from the working tools."
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The Daily Picayune had reported on the discovery one month earlier:
The Subterranean Vaults
"The excavations into the old vaults discovered behind the calaboose were continued yesterday and the chain gang were employed digging out the mud and rubbish. Nothing was found that could lead to any knowledge of their history and after digging about six feet into a strongly arched passage the workmen were stopped by an iron door, and the labor was abandoned until to-day, when the search will be continued.
"An old creole woman, upwards of a hundred years of age, it is said remembers that upon this spot stood a building in which Jesuits resided; and the most plausible supposition that can be arrived at, is, that these strong vaults were prepared as places of deposite for valuable manuscripts and other precious things, in case of war or other danger placing their establishment in jeopardy. That they were designed for some more than ordinary purpose is evident from the massive iron archings, and careful mason work used in their construction. The very fact of an apartment being built under ground in our soil, is sufficient evidence that it was designed for no common purpose; and it is also probable, although anxious precaution may have led to the construction of those vaults, that no urgent necessity ever called for their use. At any rate they should be thoroughly examined, not merely to satisfy the natural curiosity of the public, but the search should not cease while even the slightest probability exists of anything strange or secret being brought to light. If they are covered now, without their history being unfolded, they may be opened again in a succeeding century, and mystery may then assume what shape she pleases, bidding defiance to scrutiny. It will take but little labor to explore these recesses carefully, and a single relic found would amply repay investigation. They should be searched at any rate, that false constructions may not be left to perplex the public mind."
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The Editor of the New Orleans Bulletin was allowed access to the excavation site, and The New Yorker picked up his report:
Discovery of An Inquisition
"'We found that considerable progress had been made in the excavation since our visit two days previous. The water and mud were drained out by means of a fire engine so as to expose the upper section of the cell, the bottom being still covered with mire and rubbish three feet deep. On a temporary bridge of scantling we descended under the arch, so as to have a fair prospect of all that could be seen. The vault may be described as a cell arched over with brick walls and ribs of iron, about seven feet in altitude, and as many broad. On three sides, it is entirely shut in by solid masonry and iron bars.
"'The only outlet is on the side facing the South. Here a narrow arched passage opens into the vault. The floor of the passage is on the same level with that of the main apartment. The height is not so great, being about six feet, and the breadth about two feet and a half. The dimensions were large enough to permit the transit of a man of ordinary size, without difficulty. The extent of the arched recess or passage leading from the vault, has not been ascertained. It runs horizontally in a southern direction, and can be traced a distance of ten feet or more under the ground. The excavation will have to be carried on still farther before the subterranean apartments can be fully explored.'"
If you are wondering about the appearance of colonial structures, you may want to consult the University of North Carolina's Research Laboratories of Archaeology portal.
A January 1730 plan, section and elevation of New Orleans prisons has been digitized by the Archives nationales d'outre-mer here.
The old calaboose referred to above was located in the general vicinity of Exchange Alley, the Second Municipal District, Square 44.
Quoted matter above:
"Dungeons of the Inquisition in New Orleans." Christian Observer 19 March 1840. Accessed via American Periodicals database.
"The Subterranean Vaults." The Daily Picayune 19 February 1840. Accessed via America's Historical Newspapers database.
"Discovery of An Inquisition." The New Yorker 7 March 1840. Accessed via American Periodicals database.