An image of a small hole in a rock outcrop on Mars looks a bit like a doorway, so of course, all the alien enthusiasts are thinking that it IS a door - some evidence of a long-lost alien civilization.
A little misleadingly they generally show just the image with the hole, and not the surrounding rocks. Those rocks show that this is simply the most door-like of a number of similar holes where the rocks have fractured and fallen away. This one narrow image also gives an impression of something much larger.
Original images from NASA. Curiosity rover, Sol 3466
Full-sized panorama (85MB PNG, which I made with Photoshops "photostitch" tool) : https://www.metabunk.org/f/FULL PANO.png