When I stomp it, I see no echo. Therefore, there must be some dozen or so meters of solid ground below me.
Okay, explain to me why there *would* be an echo if it were not a dozen-or-so meters thick? I'm stomping (as we speak) on the floor of my apartment, which I can assure you does not extend into the bedroom of the apartment below me, and I'm not hearing any echo. Oh, and, I was hiking in this canyon a while back, and despite being made of bedrock, hitting the ground with metal hammer produced some serious echo.
You've admitted that English is not your first language, so I'll help you out: an echo occurs when sound waves travel two or more paths from source (say, where I'm stomping on the ground) to destination (say, my ears), in which some of the paths are much longer than others. You hear the sound that followed the longer path sufficiently later than that following the shorter path that you can distinguish between them. Echo. Heard, not seen. As long as an object can produce sound, and you're in a space which can reflect sound, you can hear echos.
Please restate your argument, which is currently invalid, or retract it.
-Erasmus