I used a LiPo (a decent sized one too, 5.7AH, 11.1v), underground, for ages. I never ended up running around, screaming "I'm on fire!".
As with anything, it's mitigating risk. It was padded, inside an ABS waterproof box. I only stopped using it as it was bulky as I had it, and I can source plenty of 18650's at sane prices now.
You're far more likely to set a 18650 off with a **** charger that doesn't know when to stop to be honest.
Searching for the ever elusive Underground Titty Bar.
DDDWH CC