Is the tenant responsible for damages even if it was considered an "accident"?

We have a 1945 house with the original cast iron kitchen sink. Our tenants claimed that a plastic cutting board fell from the counter into the sink and caused a hole (not just a chip) of approximately 3- 4". Is this considered 'damage' even though they claim it was not caused by their negligence? They said it just fell in and it was a fluke. The sink did have some cracks but not gaping cracks and the hole is for the most part, perpindicular to the cracks. Given that the tenants have six bicycles and two large pieces of excercise equipment, we suspect they were cleaning some metal component which slipped and caused the hole. I have a picture of the hole which shows some black scuff marks near it which are not present now. - Is this your question? Add additional information