There is a network of underground tunnels throughout the US. (no idea about the reptile people)
But by their very nature, it is easy to stop illegal traffic through them now that Trump is POTUS.
The wall prevents overland traffic from entering and exiting the US.
Mexico is a globalist run territory. Hardly a nation state anymore. Stopping all illegal travel along that border is critical to preventing the slow invasion of the US.
The dems have been using this method to import their own private army to fight legitimately armed citizens who refuse to give up the 2A.