When the Works Progress Administration erected a new United States post office for Mineola  on the corner of N. Pacific and Kirkpatrick Streets in 1936, a box containing some valuable historical documents was placed inside the cornerstone on the southwest corner.

Interested historical groups decided to open the cornerstone in celebration of the city of Mineola’s sesquicentennial, and that box and its contents will be unveiled at 10 a.m. Saturday, July 8.

The post office now serves as the Mineola Texas Historical Museum, which will house and display the box’s contents.

Police Chief Chuck Bittner and Fire Marshal David Madsen will do the honors of removing the box, following a brief program on the museum steps.

As museum board project chairman Sharon Chamblee explained, they wanted to be sure there was actually something inside the cornerstone.

A company brought in ground-penetrating radar and determined “with 90% certainty” that there is an object in the cornerstone, a grayish rectangular box about 12 by 16 inches.

Newspaper accounts from the time also confirm something was placed there.

Then they had to ensure the building would not be compromised by opening the cornerstone.

As the post office is a Registered Texas Historic Landmark, any changes to the exterior had to get state approval.

Once the box has been removed, the cornerstone will be repaired and a bronze plaque installed to include the original information along with a notation about the removal of the contents.

Eventually the museum plans to put something else inside for later generations to discover.

Chamblee explained that the decision to go ahead and open the cornerstone now was both a nod to the celebration of the city’s sesquicentennial and a desire to take steps to better preserve its contents.

It is not known what preservation methods may have been employed in 1936, so preserving the contents sooner rather than later was decided.

No information was found that said the time capsule was not to be opened for 100 years.

It will be the second time capsule to be revealed during the 150th anniversary celebration, following the opening of a time capsule planted by the Mineola High School Class of 1998 and unearthed by the Class of 2023, which then buried another capsule for opening in 2048. That tradition began in 1973 on the city’s centennial.

The cornerstone was laid Sept. 21, 1936, and the post office opened for business Feb. 23, 1937. It remained the city’s post office until 1998.

It was purchased by the city in 1999 for a future museum, which opened its doors in 2009.