Out-of-the-way and Underground

Out-of-the-way adjective - Different from the ordinary in a way that causes curiosity or suspicion.

Underground and out-of-the-way are semantically related. In some cases you can use "Underground" instead an adjective phrase "Out-of-the-way".

Underground adjective - Undertaken or done so as to escape being observed or known by others.

Out-of-the-way and underground are semantically related. Sometimes you can use "Out-of-the-way" instead an adjective "Underground".