底

1. Origins, structure, and basic meaning

• Structure: 底 (そこ) means the lowest part of a container or terrain. 箱の底に marks a location of existence; 湖の底まで marks the endpoint of reach.
• Basic usage: A photograph remains at the bottom of a box, but light fails to reach the bottom of a deep lake.

2. Common pitfalls and similar expressions

• Bottom holding an object and endpoint light cannot reach
  • に locates the photograph, while まで sets the limit of the light's reach. Reference nouns 箱 and 湖 select container interior and natural terrain.