A rat may have caused a power outage at Japan's tsunami-hit Fukushima nuclear power plant.
The Tokyo Electric Power Company (Tepco) suspects the rodent may have caused a short-circuit in a switchboard, triggering the power cut.
Two years ago a massive quake-triggered tsunami caused meltdowns at the plant.
The plant was brought under control in December 2011 and Monday's crisis was the first time since then that so many facilities had been affected by electrical failure at the same time, Tepco admits.
It said it found burn marks on a makeshift power switchboard and a 15cm dead rodent nearby.
The power company said all cooling systems were operational by early Wednesday morning.
The electrical failure shut down cooling systems for four spent fuel ponds at reactors 1, 3 and 4 on Monday evening, although cooling to the reactors themselves was not affected, the BBC reports.
The system cooling water which contained spent - but still highly radioactive - nuclear fuel rods failed and it took engineers some 30 hours to repair the damage.