Ben Otto, 37, was walking with his sons Jayden, eight, and Logan, four, near their home in Aldbury, West Midlands.
When they came across the sign for the 'Bumble Hole' nature reserve, the dad took the opportunity to stage a deviously funny sort of family photoshoot.
Otto said he pretended he was lining his boys up for the perfect shot and snapped the photo when they were completely unaware.
Jayden's head conceals the middle letters B, L and E, meaning the sign behind their heads read 'Bum Hole' instead.
Otto said his boys were in "hysterics" after realising what he had done.
"We had walked past the Netherton Tunnel a number of times and I told them we could walk through it one day, so we went there at the weekend," he said.
"The little one was scared at first because it was dark, but he soon got used to it and we came out at a nature reserve.
"I spotted the sign for 'Bumble Hole' and as soon as I saw it I thought it was a brilliant photo opportunity. Immaturity got the better of me.
"When I showed them the photo they were in stitches. Logan can't read yet because he hasn't started school but I explained what it said and he couldn't stop laughing.
"They both thought it was hilarious.
"Bum hole is Logan's favourite phrase, he even calls it me all the time. It's the naughty word I let him get away with."
The boys' mum Joanne Guidotti, who is separated from Ben, said: "When I saw the photo I thought it was great.
"It made me laugh to see the boys up to no good as usual."