Macerate

macerate
verb, transitive. To soften or break up something, especially food, by soaking in a liquid.
verb, intransitive. To become softened or broken up by soaking.

The boys looked into the opening in the cistern's lid. Jim shuddered.

"No one has cleaned this out in a long time." He glanced at Tony's face. "Do you think it's in there?"

"No telling."

Both were silent for a moment, then Tony rose and went inside. When he returned, he was carrying a long pole. Jim moved aside as his friend approached.

Tony pushed the pole into the cistern and fed it in until it struck bottom. Slowly, he began to move it about on the invisible floor, feeling for a lump the size of the box they hoped to find. Jim moved back to the opening and shone the beam of his flashlight onto the dark water inside. It was an opaque, tannin-colored soup of insect parts and leaf particles the water had macerated over who knew how many years. Even if Tony did find the box with the pole, how would they ever get it out of there?
Definitions adapted from The New Oxford American Dictionary, Oxford University Press, Inc., 2005 (eBook Edition, copyright 2008), and from Webster's Seventh New Collegiate Dictionary, G. & C. Merriam Company, Publishers, Springfield, Massachusetts, USA, 1965, depending on which is more convenient to hand.