Understanding　the　effect　of　underground　structures　on　the　seismic　responses　of　saturated　coral　sand　sites　is　crucial　for　improving　seismic　design　and　preventing　liquefaction　disasters　in　island　reef　engineering.　Despite　advancements　in　this　field,　the　specific　effects　of　underground　structures　on　the　seismic　behavior　of　coral　sand　sites　remain　underexplored.　This　study　addresses　this　issue　by　conducting　dynamic　centrifuge　shaking　table　tests　on　saturated　coral　sand　sites,　both　with　and　without　an　underground　structure.　The　seismic　responses　between　the　free　field　(FF)　and　structure　field　(SF)　were　compared　in　terms　of　pore　pressure,　acceleration,　response　spectra,　and　shear　stress-strain　behavior.　Test　results　reveal　that　the　presence　of　the　underground　structure　led　to　a　deeper　liquefaction　zone　beneath　it.　The　excess　pore　pressure　in　the　soil　layer　above　the　structure　dissipated　rapidly　after　peaking　and　completely　dissipated　by　the　end　of　the　seismic　motions.　Additionally,　the　soil　near　the　bottom　of　the　structure　exhibited　higher　dilatancy.　The　structure　amplified　the　acceleration　of　the　soil　above　and　near　its　bottom　while　reducing　the　acceleration　of　the　soil　adjacent　to　its　sidewall.　In　the　period　range　of　0.9-3.0　s,　the　presence　of　the　structure　weakened　the　attenuation　of　response　spectra　values　for　the　soil　adjacent　to　the　sidewall　and　above　the　structure.　Furthermore,　the　structure　limited　the　deformation　of　the　soil　adjacent　to　its　sidewall　and　increased　the　soil　shear　modulus.