There is a radiator fluid leak coming from a hose connected somewhere to the back heater core.
Hi. Fluid leaks are common, particularly from old rubber hoses. I suggest you have a certified mechanic, like one from YourMechanic, have all of the hoses replaced. It may seem like overkill, but all of the other hoses that are the same age can leak the same way. This would be cheaper in the long run opposed to replacing them as they burst.