DIAGNOSIS Internal leak. Most common sources are a cracked water tank connector, a loose internal hose, or a failed seal on the brewing unit's drain valve.
FIX First, rule out external spills. Ensure the drip tray is correctly inserted and not overfull. Dry everything. Run a cycle with an empty capsule or using the cleaning cycle. Watch closely to see if water drips from the seam where the body halves meet, or from near the water tank socket.
IF THAT DIDN'T WORK The leak is internal. Unplug the machine. Remove the water tank and drip tray. Tilt the machine over a sink. If water pours out, the internal drain tube from the brewing unit to the drip tray has come disconnected or the one-way valve in that tube has failed.
HEADS UP This is not a user-repairable issue. Water on the electronics will cause a short. Stop using it immediately. Contact Nespresso for service. A leak from the bottom almost always requires disassembly and part replacement.