Apparently someone once had a pond, but local residents are not sure. That could easily be the case, says urban ecologist Floris Brekelmans. Frogs can be there for years and can also live there for a long time. “It may be that there was once a passage. Or that someone had a pond. It may also be that children brought frogspawn or that the frogs walk right through the house on a warm, sultry evening when the doors are open.”