In worst-hit Aceh province in Indonesia, the tsunami drastically changed the coastline, ripping out mangroves before slamming into forests and rushing far inland, inundating shrimp farms, rice paddies and other farmland.

"The low-lying coastline disappeared, the beach disappeared, rice fields were turned into coastal lagoons," said Faizal Parish, director of the Malaysia-based Global Environment Centre.

The Aceh chapter of Indonesia's leading environmental watchdog, Wahana Lingkungan Hidup, said some 100 kilometres of the West Aceh coast was seriously eroded, and many villages were totally submerged.

"Mangrove forests, which had already dwindled with the construction of fish farming ponds on the coastline, especially on the north and northwestern coast, have practically disappeared," the group's expert Sofyan said.

But Sofyan said the worst problems were invisible ones -- soil pollution, and chemical poisoning from burst pipes of waste disposal networks.

Muamar Vebry, environment researcher with the private think tank Aceh Institute, said high levels of cadmium deposited by the sea water had made agriculture impossible in some areas.

Recent United Nations research also discovered asbestos traces in the ground, released from construction materials pulverized by the waves, which were high enough to pose a danger to human health, he said.

In Thailand, the gargantuan tsunami waves snapped coral branches and smothered fragile reefs with silt and tonnes of debris which was dragged out from the mainland as the waters receded.

About 13 percent of the coral reefs along the Andaman coastline were severely affected, and more than half of the existing resources destroyed, said Niphon Phongsuwan, marine biologist at the Phuket Marine Biological Center.

Activists complain that the replanting of the protective mangroves, which bore the brunt of the ferocious energy of the tsunami in parts of Thailand, Malaysia, Sri Lanka and India, have been haphazard at best.

"Just look at the planting of mangrove seedlings along the coast. They were in most cases done without considering the real conditions there -- just have a look, almost none of these mangroves have survived," said Sofyan.

"Regaining some of the mangrove forests along the coastline will take at least five to 10 years," predicted Parish.

Thailand too is recovering very slowly, but has enjoyed more serious efforts from the authorities and local communities toward rehabilitation.

Divers and tour operators are being educated on how to preserve the remaining reefs to avoid further human destruction.

But experts say a full recovery could take up to four years for the largely devastated Koh Mai Phai in the Phi Phi island group which was particularly hard-hit.

In the divers' paradise of the Similan islands northwest of Phuket, a massive clean-up of the reefs and sea beds was held in January, when hundreds of divers physically pushed upright fallen reefs and sea fans.

"In some areas, some rubbish still remains, such as the Phi Phi islands, there's still rubbish in the mangroves," said Sakanan Plathong, lecturer and coral reef ecologist at the Prince of Songkhla University.

But rehabilitation in Aceh -- where nearly half a million were made homeless by the tsunami -- has focused on reconstruction of houses, roads and infrastructure.

"Less visible environmental damage has remained largely ignored," Sofyan said, adding that reconstruction has proceeded without regard for the environment or human safety.

"Fish ponds and houses have been built in areas that are not fit for settlement... they are potentially liable to be hit again by a future disaster," he warned.

A reconstruction blueprint for Aceh launched with much fanfare, containing environmental guidelines to prevent a repeat of the December 26 tsunami, is being ignored by the agency overseeing reconstruction, say Sofyan and Vebry.

"The blueprint is a failure. It was... heavy on planning but was difficult to implement. It was also a failure because it was based on a top-down approach and was not action-oriented," said Vebry.

Parish said there have also been problems in convincing people to resettle elsewhere.

"The sea has gone some 250 metres to 500 metres inland, so obviously you can't go back to where you were. But people are reluctant to relocate," he said.

"And in Thailand, the authorities should not allow resorts to rebuild exactly where they were, pre-tsunami -- it's best to have a coastal green belt -- but they have."