This is also a CSS grid gallery. Since all the images are the same size, the placement is fairly easy, and they take up one cell each.
What makes this gallery special, is the 'material box' effect. The images need to have the class .materialboxed added to them, and then, when clicked on, they will expand to a larger view. One more click and they will close again. More info and ideas can be found at https://materializecss.com/media.html.
The images have been made available by Hans Top. He got his scuba license on the island of Moofushi, Maldives, and while diving, his instructor took these pictures at 16m depth. For a fantastic scuba learning experience, go visit Chuck on the Alifu Dhaalu Atoll, Maldives at this place.