DIAGNOSIS Internal leakage. The most likely source is a cracked water tank connector, a failed internal hose, or a leaking thermoblock seal.
FIX Unplug immediately. Empty the water tank. Dry the exterior. Place the machine on a dry towel. Remove the water tank and inspect the black silicone gasket at the base of the tank's connector. Re-seat it. Inspect the male connector inside the machine for cracks. Reassemble and fill the tank only one-third full. Place on dry towel, plug in, and run a brew cycle without a capsule. Observe for fresh leakage.
IF THAT DIDN'T WORK The leak is internal. The primary suspect is the thermoblock's O-ring or a water line connection that has come loose. This requires disassembly and is not user-serviceable without voiding the warranty.
HEADS UP Continuous leakage can cause electrical damage. Do not use a leaking machine. Contact Nespresso support; internal leaks are often covered under warranty for this model.