Electricity has returned to most of Spain and Portugal but with continued disruptions on the second day after an unknown failure caused a massive blackout across much of western Europe.

Electricity was restored to almost all of Spain and Portugal on Tuesday morning following a massive power cut affecting the entire Iberian Peninsula, but the cause of the blackouts remained a mystery.

That conclusion has been supported by the operator of Spain’s national grid, Red Eléctrica, which has located where outages began on Monday. Two separate incidents of power outages led to instability in the grid leading to a “breakdown in interconnections with France,” leaving the Iberian grid isolated.