dead spot

Short definition from Wiktionary

Noun

1. 1
   An area that has no radio (or wi-fi) reception.
2. 2
   A part of a scene that has insufficient lighting.
3. 3
   A location around an earthquake epicentre that is prone to earthquake.
4. 4
   The flaw of building that cannot be seen when collapsing because poor structure, built not according specified standards, or not earthquake-proof.