My first mud experience happened when I was 25 or 26...although I'd been fantasizing and jacking off to thinking about wallowing in the mud for years. I was in the military, stationed on a base in the South. I used to run a lot, and the base had acres and acres of undeveloped land. During one of my runs, I passed by a water tower. My dick got rock hard as I noticed that underneath the leaky tower was a huge expanse of thick, clay-like mud. Somehow I had the sense not to strip and fling myself into it face down. I returned the next day with a backpack containing a change of clothes and towels. Water bottles weren't so ubiquitous then, so I figured I'd just have to wipe the mud off. Looking back, someone must have patrolled out there at some point...it must have been pure dumb luck I never got caught. I returned to that mud hole many, many times during my stay there.
My first experience, though was amazing. I took off my tank and ballcap, sat down and took off my shoes. No socks. I was seriously leaking precum at this point. I circled the expanse of mud...it got slightly messier under the leak, but overall it had an incredibly tick cake batter-like consistency. It was June, hot and humid. Sweat poured down my face and chest; my pits were soaked... I turned towards the mud, ready to take my first step. As my foot sunk it into the mud, my cock got even harder. The mud was thicker and warmer than I expected...and deeper. I sunk to my knee, lost my balance and finally what I'd been dreaming of for years occurred: I went over face first, spread eagle. Mud filled my pits and cradled my crotch to the point where I was in physical pain I wanted to shoot so bad. I crawled/swam/dragged myself to the center of the mud pit where there was some shade. My shorts were pulled off of me as I did this; I was naked. Every so often a plane would fly over, but I figured I was pretty much invisible. I flipped over on my back and made a mud angel. I dipped my head under so the mud blocked my ears. I burrowed under the surface and felt that glorious warmth and pressure. I knew I was going to cum soon. I crawled/swam to a thicker part of the mud and pumped and fucked into that mud till I was almost delirious. Right as I came I plunged my face into the mud and yelled. Mud filled my mouth. When I was finally spent, I crawled out. I carefully wiped away as much as possible, but realized it was a lost cause. I pulled on my sweat pants and sweatshirt (looking back, I can't believe that no one ever asked me why I was dressed like that in the southern heat) and made my way back to the barracks. My face was still pretty dirty. I decided I would say I used mud as sun block, but no one ever asked.
I returned frequently, whoever I could. Each time was pretty much a variation of the first.