In 1911 Niven discovered ancient ruins buried beneath volcanic ash near Azcapotzalco in the Federal District, just north of Mexico City. He devoted the next two decades of his life to archeological exploration in the Valley of Mexico and through an arrangement with the Mexican government was able to fund his digging by the sale of artifacts. Niven established a private museum in Mexico City with more than 20,000 exhibits. It was later moved to Tampico.

He recovered the first in a series of unusual stone tablets bearing pictographs from his digs at San Miguel Amantla, Azcapotzalco, and elsewhere in the Valley of Mexico in 1921. This discovery eventually totaled more than 2,600 tablets and acquired notoriety through the writings of James Churchward, beginning with The Lost Continent of Mu, first published in 1926. Niven was a founding member of the New York Mineralogical Club, an honorary life member of the American Museum of Natural History, a member of the Scientific Society Antonio Alzate in Mexico, and a fellow in the American Geographic Society of New York and the Royal Society of Arts in London. In 1929 he moved to Houston, where he donated a large number of Mexican artifacts to the new Houston Museum and Scientific Society and served on its board of trustees. In 1931 he moved to Austin. He died there on June 2, 1937, and was buried in Mount Calvary cemetery.

His later discovery of twenty-six hundred inscribed stone tablets in the Valley of Mexico aroused considerable controversy, and inspired James Churchward to put forth an interpretation of the origins of the Native Americans in The Lost Continent of Mu (1926). They remain controversial to this day.

Niven's upper city was built before the mountains were raised at the beginning of the Pleistocene; his lowest city was built tens of thousands of years before this and goes far back into the Tertiary Era. Niven’s Mexican Bird Tablets . Among Niven’s Collection of 2600 ancient stone tablets, there are about thirty containing birds.

“Over an area of about 200 square miles in the Valley of Mexico from Texcoco to Haluepantla, yhere are hundreds yes thousands of clay pits.

After serving the City of Mexico as a source for building materials for more than 300 years, these pits have enabled me to make an extensive examination of a vast ruin. Resently my efforts have been rewarded with some remarkable and startling discoveries, which seem to open up a new field for archelogical research on this continent.

My Operations have been confined to an area some 20 miles long by 10 miles wide, in the northwestern portion of the great valley. There I have found traces of two civilizations and three well preserved concrete floors or pavements , each one aat some time underlying a large city. These pavements are at a depth of from 6 to 21 feet from the surface. Above the first there is a deposit of small boulders, pebbles and sand covered with a foot thick coating of the rich soil of the valley. The great age of this upper or younger floor must be plain, when every layman stopss to consider the number of years required to deposit one foot of earth on a level plain. Everywhere in this deposit of boulders, pebbles and sand above the first floor I found fragments of broken pottery, small clay figures, diorite beads, spear and arrow heads, spindle whorls and other artifacts, mostly broken.

The second concrete floor is from 4 to 6 feet below the first, the difference in distance between the two accounted for by the broken condition of the lower pavement, due probably, to seismic disturbances. In the intervening space between the two pavements, one and two, I have failed to find a single piece of pottery, or any other trace to indicate that people had lived there.

Underneath the second pavement, however came the great find of my many years’ work in Mexican archaeology. First I came upon a well define layer of ashes from two to three feet in thickness , and since proved by analysis to be of volcanic origin. Just below the ashes I found traces of innumerable buildings , large, but regular in size, and appearing uniformly in more than 100 clay pits , which I have examined during my recent investigations.

All of these houses are badly ruined, crushed and filled with ashes and debris. In the past week’s work I found a wooden door, the wood of which had petrified and turned to stone. The door was arched with a semicircle lintel, maid by bending the trunk of a tree about five inches in diameter or thickness. This is he first curved arch ever found in the ruins of Mexico; and , as the walls of the house were laid of stone, bound together with white cement, harder ttthan the stone itself, this wooden arch must have been put in as an ornament. Cutting through the door, I came into a room about 30 feet square, filled with almost pure volcanic ash, apparently about the only room strong enough to withstand the terrible weight of soil , ashes and stone above it. The roof which had been of concrete and stone, and flat, had caved in, but around the lower edges of the room great flat fragments of this roof had formed arches, little caves in the ashes, in which were preserved many artifacts of the dead race. With the artifacts were bones, numberless bones of human beings, which crumbled to the touch like slacked lime.

Above their tomb the waters of a great flood had raged, wiping out another civilization. Flood and the crashing boulders had not disturbed the sleep of this mighty race.

The doorway was over six foot deep, and on the floor, thirteen feet from the door, I came upon a complete gold-smith’s outfit. It consists of a terra-cotta chimney 25 inches in height, tapering upwards from a round furnace 15 inches in diameter. On the floor around the furnace, to which still adhered bits of pure gold, I found more than 200 models, which had once been baked clay, but which had been transformed into stone. All of these were duplications carved on figures and idols which I found later in the same house. Evidently this had been the house of a prosperous goldsmith and jeweler of the better class in the ruined city.

Some of the models or patterns were less than one-twentieth of an inch in thickness, and were used for the manufacture of the gold , silver and copper dress, head, breast, arm and ankle ornaments which thw statuettes show the people to have worn in those days. Each model was thickly coated with iron oxide, bright and yellow, probably put on there to prevent the molten metals adhering to the pattern while in the casting pot…The work is fine, beautifully polished, and shows a hight of civilization fully as great, if not greater, than that possessed by the Azteca when the Spanish under Hernando Cortez first invaded Mexico.

But what struck me most as the feature of the room was the mural decorations. Evidently there had been a slight partition through the center, while from the rear walls the dim outline of the door appeared to lead into another room, which is now so complete a ruin that I doubt that anything other than bones will be found in it. Here are wall paintings done in red, blue, yellow, green and black, which compare favorably with the best photographs I have ever seen of Greek, Etruscan, or Egyptian works of the same kind.

The ground color of the wall was pale blue, while six inches down from the fourteen foot ceiling a frieze painted in dark red and black ran all around the four sides. This frieze owing to the fact that it had been glazed after painting, wit a sort of native wax , is perfectly preserved, so far as colors and patterns go. It has been, however, broken in three places by fragments of the falling roof, but otherwise it is almost as legible as the day when first painted. It depicts the life of some person evidently a shepherd, bringing him from babyhood to his death bead.

Beneath the room I found the tomb of some one of importance, possibly of him whose life was portrayed in the frieze above. In this vault, which was only three feet in depth and lined with cement, were seventy five pieces of bone, all that remained of a complete skeleton. One large fragment of the skull contained the blade of a hammered copper ax, which had evidently dealt death to the occupant of the tomb, and which had not been removed by his relatives or friends. The bones crumbled to the touch, so long had they been in the tomb, but there were other objects more interesting than the bones. One hundred and twenty-five small clay terra- cotta idols, manikins, images and dishes of all kinds were ranged around the bottom of the tomb. Among the objects unearthed were over 2600 stone tablets.