Gold's concept, which focuses on an alternative explanation for oil and mineral deposits, was published in his book "The Deep Hot Biosphere."

It's still uncertain, but logical, that there could be a large biochemical system deep in the Earth that would work better at high temperatures and high pressures," Gold said.

Others are skeptical. Dr. Harold Klein, who led the Viking lander project team that searched for signs of life on Mars in the 1970s, pointed to silicon as far inferior to carbon in forming the complex polymers crucial to life.

"Personally, I doubt the concept of silicon-based life. If we find living organisms inside the Earth, I bet they will be carbon-based," Klein said.

Nevertheless, Dr. Klein encourages future Mars missions to carry an instrument to test non-carbon-based organisms, just in case. According to David Noever, a scientist at NASA's Astrobiology Institute, it's possible that silicon's chemical properties are sufficiently altered by the high temperatures and pressures deep within the Earth to make it more adaptable to forming complex molecules.

He said some scientists at the US space agency were taking the idea of silicon-based life seriously, especially when it came to the search for extraterrestrial life.

"It's almost naive to assume that all life has to be carbon-based. Perhaps I could cite good cases for life based on both silicon and phosphorus," Noever said.

According to Dr. David Williams, a researcher at the Natural History Museum in London, silicon is used by some carbon-based single-celled organisms called diatoms to form protective shells. However, diatoms are essentially carbon-based.

In recent years, however, strange organisms have been found deep within the Earth's crust. Steve Jones, professor of genetics at the University of London, said: "There's an unknown universe down there that has evolved organisms with such strange metabolisms that, compared to humans and fungi, they're identical. God only knows what else will be found there."

Microbes have been found on the ocean floor, at depths and temperatures where living organisms were previously not expected to be found.According to Dr. Harry Elderfield, an Earth scientist at the University of Cambridge, without knowing what silicon-based living organisms might be, it is impossible to predict how scientists might test them.

Gold has now been described by Stephen Jay Gould, president of the American Association for the Advancement of Science, as one of the most iconoclastic scientists – but not everyone is always right.