Named for a holy shrine in Madrid, the heavily armed galleon Nuestra Señora de Atocha served as the almirante (or rear guard) of the Spanish fleet that left Havana in early September 1622. In addition to 265 people, the ship carried as much as 40 tons of silver, gold and assorted riches from Colombia, Peru and other regions of South America. After a hurricane struck on September 5, 1622, the eight other ships in the fleet sank, littering the ocean floor from the Marquesas Keys to the Dry Tortugas, between 30 and 70 miles to the west of Key West, Florida. Two sailors and three slaves aboard Atocha survived by clinging to the ship’s mizzen, the only part that remained above water, but rescuers were unable to open the ship’s hatches. A second hurricane on October 5 further destroyed the wreck, and despite six decades of searching by Spanish salvagers, no trace of Atocha or its treasures would be found.

Flash-forward to the late 20th century, when a former chicken farmer turned shipwreck- and treasure-hunter named Mel Fisher. Beginning in 1969, Fisher searched relentlessly for Atocha, making small discoveries along the way (three silver bars in 1973; five bronze cannon in 1975) that convinced him he was getting closer to the ship itself. (Tragically, Fisher’s son Dirk, his wife and another diver died when a salvage boat capsized soon after the cannon discovery.) By 1980, Fisher’s team had also discovered a significant part of the wreck of Atocha’s sister ship, Santa Margarita. Finally, in July 1985, Fisher’s son Kane sent a message to his father’s headquarters: “Put away the charts; we’ve found the main pile!”

In addition to a fortune’s worth of gold and silver bars, coins and jewelry, the bounty recovered from Atocha included emeralds traced to a mine in Colombia, along with items ranging from navigational instruments to ceramic vessels, all offering a glimpse into 17th-century life in Spain and the New World. With an estimated worth of some $400 million, the Atocha treasure made Fisher, his family members and other investors millionaires. Thanks to efforts by historians and archaeologists as well as environmentalists, Fisher’s success led to reforms in the laws governing shipwrecks and salvage. In 1987, Congress passed the Abandoned Shipwreck Act, which gave states the rights to shipwrecks located within three miles of the coastline.

After the discovery, items from the cache of treasure went on permanent display at the Mel Fisher Maritime Museum in Key West, Florida. Now, some 40 items from the Atocha and Santa Margarita yield will go on the block at the auction house Guernsey’s in New York City early next month—August 5, to be exact. According to Fisher’s daughter, Taffy Fisher Abt, the lots offered for sale will include some of her parents’ favorite pieces. (Fisher died in 1998, while his wife, Dorothy, passed away in 2009.)

Mel Fisher wore one of the pieces to be auctioned off—a heavy gold chain that hangs past waist-length—when he appeared on “The Tonight Show with Johnny Carson” soon after discovering the Atocha’s treasures. Dubbed “the money chain,” it consists of individual links, each around the size of a thumbnail, which in the 17th century could have be removed and used as formal currency. (At the time, the Spanish king had placed a 20 percent tariff on gold bullion, known as the Royal Fifth, but the tax didn’t apply if the gold was turned into jewelry.) According to pre-sale estimates, the chain could fetch some $90,000 to $120,000 at auction.

According to her daughter, Dorothy Fisher favored a knee-length gold chain with ornately carved links; that item could fetch around $40,000 to $50,000. Another item, a gold-and-enamel spoon of Peruvian and Spanish origin, is believed to have been used during Communion by Catholic priests sent to the New World to convert the native population. Among the intricate designs carved along the spoon’s neck is a masculine face between a pair of condors, an Inca symbol of royalty. The spoon is expected to go for some $160,000 to $180,000.

Among the more intriguing items recovered from Atocha were a number of bezoar stones, egg-sized objects made of organic material found in the digestive tracts of llamas, alpacas, deer, sheep or other two-stomached animals (known as ruminants). When dipped into a cup of liquid, bezoar stones were thought to remove any toxins or poisons from that liquid—a necessity for rich and powerful 17th-century individuals worried about servants or rivals adding arsenic to their wine goblets. One of the stones, mounted in a gold setting and designed to dangle from a chain, will be part of the auction; pre-sale estimates indicate it could fetch as much as $28,000 to $35,000.