Hex come from a planet of constant storms and hail, and crawling underground is a great way to escape that. They do not create the tunnels, they can only dig around as quickly as we can, but they can squeeze through anything that their shoulders fit through. The bioluminescence + tapetum lucidum wombo combo helps with seeing without any lights. Because they are their own lights.

Though in the modern era crawling through tunnels like this is either for recreational caving or for escaping hail while out in the wilderness, they have buildings and underground tunnels and stuff so squeezing through like this isn’t like... The daily experience or anything but yeah.