Fresh from his journey to the deepest point of the Pacific in a solo submarine dive, Hollywood director James Cameron has spoken to Australian schoolchildren, answering questions on how fast his craft could travel to how he went to the toilet.

The "Titanic" filmmaker, who last month plunged seven miles (11 kilometres) to the bottom of the Mariana Trench in the western Pacific, told students at Perth's Roseworth Primary School that science had been his favourite subject.

In particular, he was intrigued by astronomy, the Perth Now website reported him as saying during a video chat Tuesday via satellite from a ship lying in the Pacific off the coast of Guam.

"I knew I was probably not going to be an astronaut and travel to another planet, but I knew there was something very much like another planet right here on earth, and that's underwater," Cameron said.

"Sometimes when you're on a coral reef or a deep-sea dive, you see animals that are so colourful, so amazing and so different to anything you could have imagined, then you realise there is an alien world here on earth."

Speaking simultaneously to the school in Perth and one in California, he said the marine life he had seen was "nature's imagination".

After taking his submarine dive to the bottom of the Mariana Trench, where temperatures are barely above freezing and the pressure is a crushing thousand times that at sea level, Cameron described the landscape as "lunar".

Cameron spent about two and a half hours at the bottom of the ocean, but cut short the planned stay of six hours because of problems with his specially designed submersible's hydraulics system.

Questions from the students, aged from about nine to 12, ranged from one about how fast the submarine could travel, to the personal: "How did you go to the toilet in the sub?"

As the students giggled, he reportedly replied: "Have you ever gone on a really long car trip with your family and they keep a bottle for you so you don't have to pull over?"

Cameron's voyage to the valley of the Mariana Trench, which lies southwest of Guam, was the first manned expedition to the trench in more than half a century and the culmination of more than seven years of planning by the director.