Strange you should say that.
Radar, as everyone knows, is the reflection of Elecromagnetic radiation (radio signals) off a surface many yards or miles away.
Radar doesn't work through solid rock,
brickwork or bridges.
What radar needs to detect an object, is a reflective surface on that object. Rounded objects, other aircraft with smooth circular fuselages are no problem.
'Radar invisible' objects:- those with multi-faceted surfaces like the Stealth fighter, are a bit more tricky to see a reflection because there ( hopefully ) won't be a surface perpendicular to the direction of the signals.
Other 'Radar invisible' objects include 'Surface hugging' missiles such as the one that hit HMS Sheffield. The radar signal bounces off the sea surface and is lost into the outer atmosphere.
Another 'radar invisible' object is the "Large Triangular' object often seen over Area 51, Nevada.