Sinkholes are known in Florida for swallowing up houses and highways. Florida is made up of sand and limestone, and in times of rain huge caverns form in the limestone to fill with excess water. Then, when there is a drought, all that underground water flows away leaving the cavern empty and unsupported. The roof of the cavern collapses, and the size of the empty cavern determines how many houses it eats.

In the marsh water will rapidly fill in any new depression, but every now and then you can see a small sinkhole like this one in a dry area

Since this is in a secluded spot there is no danger of anyone walking into it - or of it swallowing a house. The dirt and vegetation will soon fill in the hole, and there will be no sign it was ever there ...unless a tree falls over to mark the spot!