Enhancing　boiling　heat　transfer　and　elaborating　underlying　mechanisms　are　of　great　significance　to　assist　in　improving　the　efficiency　of　energy　systems　and　solving　thermal　management　bottlenecks　in　advanced　electronics.　Numerous　experimental　studies　have　proved　nanostructured　surfaces　enable　boiling　heat　transfer　to　be　enhanced　strikingly.　Nevertheless,　it　is　a　great　challenge　to　perform　more　in-depth　research　due　to　the　limitation　of　the　observation　accuracy　of　conventional　experimental　approaches.　To　optimize　designs　of　nanostructured　boiling　surfaces　and　fully　reveal　the　microscopic　mechanisms,　the　aspect-ratio　effects　of　rectangular　nano-cavities　on　nucleate　boiling　heat　transfer　are　investigated　systematically　by　molecular　dynamics　simulations　in　this　study.　The　nano-cavities　have　the　same　width　of　3　nm　and　different　depths　of　2　nm　to　8　nm,　corresponding　to　aspect　ratios　from　2:3　to　8:3.　The　results　illustrate　that,　compared　with　the　plain　surface,　concave　nanostructured　surfaces　can　cause　a　significant　heat　accumulation　effect,　resulting　in　remarkable　reinforcements　in　absorbed　heat　flux　and　bubble　nucleation.　The　nucleate　boiling　heat　transfer　enhancements　originating　from　large-aspect-ratio　nano-cavities　are　fully　verified　by　the　notable　reductions　in　the　incipient　nucleation　time　and　boiling　initiation　temperature.　Attractively,　for　cavities　with　the　same　width,　an　optimal　depth　for　maximal　nucleate　boiling　enhancement　can　be　obtained,　which　is　7　nm　in　the　present　study.　In　this　case,　the　incipient　nucleation　time　is　about　eight　times　shorter　than　that　for　the　plain　substrate　and　the　decrease　in　boiling　initiation　temperature　is　up　to　53　K.　Moreover,　a　novel　and　straightforward　insight　into　the　microscopic　enhanced　mechanism　has　been　proposed.　It　is　elucidated　thoroughly　from　the　perspective　of　the　thermal　energy　accumulation　of　liquid　at　the　initial　nucleation　site　by　taking　into　account　the　heat　absorbed　from　the　solid　wall　and　the　heat　transferred　to　the　adjacent　cryogenic　liquid　comprehensively.　These　analysis　and　simulation　results　are　in　great　agreement.　This　study　provides　significant　guidance　for　state-of-the-art　two-phase　thermal　management　applications.(c)　2022　Elsevier　Ltd.　All　rights　reserved.