my house was built in 1960. I have lived here 4 years. all the plumbing to my knowledge is hooked up to?

city sewer. however, i've noticed when i run my washing machine a puddle forms outside next to the house. I dug about 2 feet and there is a cement block? i dont thinkits a septic because it looks like there is no opening at the top. Everytime i run the wash the puddle forms and then slowly will drain away. I live in NJ and the house was built in 1960..what am i dealing with here? thanks