But when authorities in Fontana lifted it up, they made a shocking discovery - it was a trapdoor opening into a secret underground hiding place.
But wait, there was more.
Beneath that hiding place was another hidden level being used as a shooting range. On that level, members of Fontana Police Department's SMASH unit found thousands of rounds of ammunition, including a 100-round drum for an AR-15 rifle.
The operation started out as a search of the residence of a known gang member on Thursday night, Fontana Police said, adding that they booked the suspects at the residence and took the ammunition.
Fontana Police couldn't resist leaving a cheeky message for evildoers on their Facebook page - "We strongly recommend that you put as much effort as you did in your underground cave, into becoming a productive member of society. You will thank us later for this advice."