DIAGNOSIS Internal leak. The most likely culprit is a cracked or disconnected water tube inside the reservoir, or a failed seal on the heating element assembly.
FIX Unplug it immediately and empty all water. Remove the water reservoir (usually lifts straight up) and inspect the small silicone tube at the bottom that feeds water to the pump. Make sure it's securely connected to the nipple. If it's cracked or loose, that's your leak source.
IF THAT DIDN'T WORK The leak could be from the heating element gasket. You'd see mineral scale or rust around the metal boiler plate inside the machine. This is a more serious fix that typically requires disassembly and a new seal, which isn't recommended for most users.
HEADS UP Never run a machine you know is leaking from the base. Water hitting the electrical components is a serious shock and fire hazard. If the internal tube looks fine, it's usually more cost-effective to replace the whole unit.