The bottom couple of feet of two steel 6 x 6 posts embedded in concrete. Part of each one has been cut away to create a wider gap than usual between them. My boot is sticking into the lower part of the frame to give the picture of some scale. The opening appears to be about 10" x 12" high.
https://files.sfba.social/media_attachments/files/113/829/729/060/326/570/original/92cf8d0d1ebf40c1.jpeg