Castillo de San Marcos - A special 'cannon-swallowing' fortress in the US

In 1702, when Spain was still dominating Florida, a British legion from the Carolinas advanced towards Castillo de San Marcos, a Spanish fortress on the Atlantic coast. This fortress protected the trade routes of the Spanish Empire as well as the town of St. Augustine, and the British wanted to occupy this important political and economic position.

Led by Governor James Moore, the British ship dropped anchor and began the attack. But after nearly 2 months of shelling and gunfire, the fortress still stood. In fact, the fortress wall "swallows" cannon bullets. How the walls were able to do so was a mystery for the next three decades.

Normally, cannon bullets will crack rocks in a radius around the contact area, causing heavy damage to the structure. However, the walls of Castillo de San Marcos are not like that. They were built from coquina, a sedimentary rock formed from the shells of dead marine animals, making the walls less vulnerable to British attacks. As one British soldier described, the stone wall "didn't crack, but sank in, making room for cannonballs, as if a knife had been inserted into a piece of cheese".

British troops vented their anger on the town of St. Augustine, and the townspeople hid in the fortress. Historian Susan Parker at Flagler University (St. Augustine, Florida, USA) said: "The British captured the town, but failed to take down the fort. So they burned the town before withdrawing." The British returned to attack the fort again in 1740, but to no avail.

In 2015, more than 300 years after the fort was built, a team of scientists from the University of Florida and the US Army conducted research into coquina's physical properties to find out how it withstands the force of impact. touch.

After two years of testing, the results showed that although coquina looked like sandstone, it acted like sponge. In combat, sandstone walls will be smashed to pieces, but coquina has the rare ability to absorb mechanical stress thanks to its rather fluid internal texture.

Although the animal shells that make up coquina have been stacked and pressed for thousands of years, they do not form blocks like concrete but can move slightly. Thus, when a cannonball hits the coquina wall at Castillo de San Marcos, it shatters the parts of the shell that hit it directly, but the surrounding parts make room for the bullet itself.

It is unclear whether the Spaniards knew about the properties of coquina when building the fortress walls. The stone was obtained from a neighboring mine, present-day Anastasia National Park. But apparently, they knew how to take advantage of this feature and turn the wall into a training ground.

Castillo San Marcos is the oldest masonry structure in the United States and is one of only two rare buildings in the world built with coquina. During its operation, this fortress was never captured by arms, partly due to special materials. This place was removed from the fortress list in 1900 and became a national monument in the United States in 1924.