An elderly man fell to his death after slipping down a “lava tube” in his yard in Hawaii, local police said Wednesday.

Authorities responded to man’s house in Hilo on Monday morning after an acquaintance of the man said he had not been heard from in several days, Hawaii police said.

A lava tube skylight.

A lava tube skylight. (Tim Orr, U.S. Geological Survey)

Investigators determined he had fallen through a soft area of ground in his yard, then plummeted down a lava tube, authorities said.

Lava tubes are large conduits beneath the ground that are formed by flowing lava. When they’re extinct, the tubes do not have lava in them, but form large tunnels beneath the earth’s surface.

Fire department crews pulled the man’s body from about 22 feet beneath the ground, police said.

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An autopsy determined the man died from the fall down the tube, authorities said.