Their model consists of two concentric steel cylinders, three metres high and 80 centimetres in diameter, filled with molten sodium.

BBC: Planet Earth

Conventional optical fibres (the most common form of waveguide at the moment) generally consist of two concentric layers of glass.

ECONOMIST: New-age crystals

The radiation belt, which contains "two concentric rings of charged particles trapped by Earth's magnetic fields" doesn't affect current astronauts going to and from space because they are moving too fast to be hit by the radiation.

ENGADGET: Space elevator ride may kill humans due to ionizing radiation