The Idol will be found in a small square chamber that appears empty.
However, as soon as the Idol is lifted, heavy stone doors slide shut on either side and a spiked ceiling slowly descends to crush Spelunky Guy.
If a hole is blown in the floor, be aware that spikes will descend through the hole until they hit something solid.
It is also possible to place a Pushable Block in the doorway before retrieving the Idol, which prevents the door from closing fully. You can then push the block away and the door will remain half-open, but this obviously requires a conveniently placed push block.
Note that the underside of the block ceiling of this trap always deals death when contacted, even before the spikes have been revealed.
If you touch the underside while flying with a jetpack or exploring with climbing gloves, you will be killed in spite of its harmless appearance.
The obligatory booby trap cliché that is the 'inescapable descending spikes of death' is a reference to the trap popularised in Indiana Jones and the Temple of Doom, the thematic inspiration for Spelunky.