Back in my early days of occasionally doing things with less intelligence, I was inspecting a house that turned out to have three different crawl spaces.
The house was a long rambler with a middle crawl space and crawl spaces on the east and west sides. There was a hatch for the west crawl space, and I traversed that one. There was a hatch for east crawl space, and I traversed that one. While in the east crawl space I noticed that the access to the middle crawl space was just a tunnel dug out under the foundation. Being more in the action mode then the thinking mode, I went through. However, to get through it turned out I had to lay on my back, exhale all my air, use my legs to push myself through and slide up the tunnel into the middle crawl space. Mission accomplished. Only issue was that it was also the only way out, so the same process had to occur to get out of the crawl space.
All three crawl spaces were about 18 inches high and pitch dark with only the light from my flashlight. This short story was written for all you claustrophobic people.