The hunt for the Clotilda, the last American slave ship, continues.

The Alabama Historical Commission, Search Inc., the Slave Wrecks Project, and a team from National Geographic spent five days cataloguing historic shipwrecks in the vicinity of 12-Mile Island in the Mobile-Tensaw Delta.

The team first spent several days "mowing the lawn" with survey equipment. The term refers to a methodical sweep of the river, from bank to bank, with an array of sensors. Side-scan sonar was used to discover portions of shipwrecks that lie exposed on the river bottom, while a magnetometer was used to detect metal objects in the river, including hidden under the mud. (Old wooden ships are actually full of metal in the form of wrought iron nails and fasteners, and give a powerful signature on the magnetometer.) Finally, a sub-bottom profiler was used to study the dimensions of objects buried by mud. That machine can see through the mud and paint an image of things buried below.

After the survey was completed, divers explored several wrecks that could possibly be the Clotilda. The team also dove several wrecks that were clearly not the Clotilda, but were interesting from an archaeological perspective, such as an iron-hulled sailing ship.

Results from the investigation will not be revealed publicly until a final report is prepared. The National Geographic Society funded part of the expedition and asked the state and archaeologists with Search Inc., to sign Non-Disclosure Agreements regarding the research.

In addition to reporters from National Geographic Magazine and the company's digital platform, the research group included Fredrik Hiebert, archaeologist-in-residence with the National Geographic Society. Hiebert has been involved in most of National Geographic's recent archaeological blockbusters, including the exploration of Christ's tomb in Jerusalem and the search for hidden chambers in King Tut's tomb in Egypt's Valley of the Kings.

Hiebert was in Alabama to operate the Geographic Society's state of the art sub-bottom profiler machine. Data from that machine will be key to efforts to find the final resting place of the Clotilda.

"One of the things about being an archaeologist is we get to tell the untold story. That's certainly one of the reasons National Geographic is interested in the epic story of Africatown and Clotilda," Hiebert said. "It's a story that needs to be told on a national and international stage. Everyone is going to be interested. I hope it will now be included in the pages of textbooks. Most high school and middle school textbooks don't tell the Clotilda story. Even here in Alabama. If there is anything we can do with the people of Africatown, the people of Mobile, of Alabama, it is get this story to the public."

The overall exploration was borne of efforts by AL.com to locate the remains of the Clotilda in January. The discovery of the first wreck identified by AL.com resulted in a February expedition that included experts from the Slave Wrecks Project, the Smithsonian Museum, the National Park Service, the Alabama Historical Commission, Diving with a Purpose, and Search Inc. That wreck was revealed to be from the right era based on construction techniques typical of the 1850s, but was too long to be Clotilda. More recently, analysis of the wood on that ship revealed that it is made of Douglas fir, which grows on the Pacific Coast, meaning the ship was built out west and sailed around the tip of South America to reach the Gulf.

Kamau Sadiki, of the Slave Wrecks Project and Diving with a Purpose, participated in the February exploration of the first ship suspected of being the Clotilda and the most recent search. He said it was hard to overestimate the power of finding the wreck, and that its discovery would begin a healing process in Alabama and Benin, where the Clotilda captives were stolen from.

"Searching for the Clotilda is a significant step in restoring historical memory, and reconnecting the descendent communities of Africatown and Benin," Sadiki said.

As soon as the archaeologists concluded that first ship couldn't be the Clotilda, AL.com commissioned the University of Southern Mississippi to search the river with side-scan sonar, a magnetometer, and a sub-bottom profiler. That search revealed a number of possible candidates for the final resting spot of the Clotilda.

In April, a team from AL.com, the University of Southern Mississippi, and Underwater Works Dive Shop, explored several wrecks revealed by the Mississippi survey. The group informed the Historical Commission that it had located another wooden wreck from the 1800s near Twelve Mile Island.

The next day, the Alabama Historical Commission announced that Search Inc., had been hired to survey the river and hunt for the Clotilda. Soon after that, the participation of the National Geographic Society was announced. Alabama Historical Commission's State Archaeologist Stacye Hathorn was also part of the team.

James Delgado, a celebrated marine archaeologist with Search Inc., said the data provided by the University of Southern Mississippi enabled his team to concentrate their efforts on likely targets, rather than starting from scratch. He pointed to the 1800s wreck found using the Southern Miss data.

"That's the kind of target we'll really paint hard with the National Geographic sub-bottom profiler," Delgado said. "Because what you're really looking for is length, beam, all of the things that we can measure."

The Clotilda's measurements are well documented, though the ship was reportedly burned at the end of its final voyage in 1858. The hunt for the ship has inspired numerous searchers over the years, hoping to put a capstone on one of the most infamous voyages in maritime history. Though it had been illegal to import slaves for 50 years, a Mobile plantation owner and steamboat captain, Timothy Meaher, decided to bring a cargo of captured Africans in through the port of Mobile to win a $100 bet. You can read a more thorough description of the ensuing journey here.

Fearing discovery, Meaher and the boat's captain, William Foster, decided to burn the Clotilda after completion of its voyage. Foster later stated that he burned the ship near Twelve Mile Island in the Mobile River.

"Working with what Southern Miss did with you, adding to that and augmenting that, we have been able to bring a really high degree of resolution to our imaging. And then the sub-bottom data builds onto that. This is a systematic approach. We do the low, slow mowing, just back and forth across the river, and then comes the diving," Delgado said. "The university data was first rate. They did a really good job with the bathymetry. We are merging all the data together, theirs and what we collect. I believe this is the first time this stretch of the river has ever been surveyed with modern equipment. It has a story to tell."

The divers got in the water on Day 3 of the search. The first ship they dove on was known not to be the Clotilda. AL.com and Southern Mississippi had previously explored that wreck, which turned out to be an iron sailing ship with a deck made of wooden planking. It, like a number of the wrecks around 12 Mile Island, was likely scuttled up the river at the end of its useful life. Delgado said sailing vessels made of iron like this one - which probably dates sometime between 1870 and 1900 -- were only produced for a short time due to rapid advancements in engine technology that made sailing vessels obsolete.

The final two days were spent investigating a few wrecks more thoroughly, including the 1800s wreck identified in April. The targets for the last two days of diving were selected based in part on imaging from the sub-bottom profiler.

After the data collected during the expedition are analyzed, Search will submit a report to the state. That report will likely call for further exploration of the most likely candidates.

"Making a positive identification of a wreck is a difficult if not tricky process not unlike solving a CSI case. Finding an identity involves detailed study, collecting forensic evidence, and then systematically and aggressively questioning not why a wreck might be a certain ship but why it cannot be," Delgado said. "Finding Clotilda is one goal of this survey, which is focused on documenting everything we can find that has come to rest in this graveyard of ships, but it will take time to sift through the data, conduct laboratory study and do additional research before we can offer a scientific opinion on a possible Clotilda site. Further study, such as a detailed excavation, might be required. We know that some of the wreck sites found are not Clotilda, but even with that, we have yet to put a name to any of them."

Lisa Jones, executive director of the Alabama Historical Commission thanked the group the state assembled to hunt for the long-lost ship.

"Our partners have a vested interest in documenting cultural resources in portions of the Mobile River as well as reconnecting the story of Clotilda to a national and international audience," Jones said. "This is a story with profound meaning in Alabama, and especially to the descendant community of Africatown."

Ben Raines specializes in investigations and natural wonders. You can follow him via Facebook, Twitter at BenHRaines, and on Instagram. You can reach him via email at braines@al.com.

You can watch Ben's most recent documentary, The Underwater Forest, here on Youtube.