Seven years after a massive Japanese earthquake and tsunami claimed 18,000 lives and triggered the Fukushima nuclear disaster many communities remain ghost towns.

The 2011 tsunami was triggered by a magnitude 9.0 earthquake under the Pacific Ocean.

The killer tsunami left about 18,500 people dead or missing and swept away entire communities as it rushed inland.

On Sunday, residents along the coast gathered outdoors to remember the tragedy as sirens wailed at 2.46pm, the moment the magnitude-9.0 offshore earthquake that set off the tsunami struck on March 11, 2011.

A deserted school in the Fukushima prefecture. (AP). (AP)

The tsunami overwhelmed sea walls and washed away buildings, cars and entire neighborhoods as it swept inland.

It knocked out power at the seaside Fukushima Dai-ichi nuclear power plant, causing partial meltdowns in three reactors.

Prime Minister Shinzo Abe said at an official ceremony in Tokyo that reconstruction is making steady progress, but more than 70,000 people are still displaced and many have no prospect of returning to their homes.

Prince Akishino, the second son of Japanese Emperor Akihito, expressed hope that the tsunami would raise awareness and help prevent or mitigate damage from future natural disasters.

After seven years wild animals including boars roam the area. (AP). (AP/AAP)

“It is my earnest hope … that we hand down the knowledge to future generations in order to protect many people from the dangers of disasters,” he said.

Cleaning up the still-radioactive Fukushima nuclear plant site remains a daunting challenge that is expected to take 30 to 40 years.

A government-commissioned group of experts concluded last Wednesday that a costly underground ice wall is only partially effective in reducing the ever-growing amount of contaminated water at Japan's destroyed Fukushima nuclear plant, and said other measures are needed as well.

A resident remembers the devastating tsunami of 2011 that hit coastal Japan. (AP).

The plant's operator, Tokyo Electric Power Co., says the ice wall has helped reduce the radioactive water by half. The plant also pumps out several times as much groundwater before it reaches the reactors via a conventional drainage system using dozens of wells dug around the area.

The groundwater mixes with radioactive water leaking from the damaged reactors.

Contaminated water also results from rainwater that comes in contact with tainted soil and structures at the plant, which suffered meltdowns of three reactors after a March 2011 earthquake and tsunami.

Radioactive checks are carried out around the affected power plant. (AP). (AP/AAP)

The panel agreed that the ice wall helps, but said it doesn't completely solve the problem. Panel members suggested that additional measures be taken to minimize the inflow of rainwater and groundwater, such as repairing roofs and other damaged parts of buildings. \