system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know for sure that there is molten lava in earth’s core?
gpt
Earthquakes and some other events like nuclear weapon explosions set off seismographs. These record the pattern of the ground waves that reach it. Earthquakes create 2 different kinds of waves which travel at different speeds. Sometimes the direct line from the seismograph to the event goes through the earth. When this happens, they can use the timing and data from multiple seismographs in different locations to make conclusions about the material inside the earth.

Also lava.

Edit: not lava.