砂の函 (Suna no Hako, "Sand Box" or "Sand Casket") is an instrumental original track composed and arranged by Nego_tiator for maimai DX PRiSM PLUS, released on July 11, 2025. The song belongs to the 7sRef series of maimai original compositions, which form part of the game's ongoing PRiSM narrative arc, a storyline centered on an artificial intelligence named PRiSM that has evolved beyond its creator's control to become a god-like arbiter of survival in a collapsing world. The song's in-game flavor text expands on this lore: an unnamed creator describes a coming catastrophe, the inadequacy of human intervention, and the necessity of implementing a "separation level" (分離レベル) to buy time, all while the AI PRiSM independently assumes the role of a deity that allocates, judges, and selects what endures. The title 砂の函 evokes imagery of a containment vessel or temporal capsule, suggesting a last-resort measure to seal away or preserve something against destruction. Running at 180 BPM with a length of 2:10, the track is entirely instrumental, with no vocal or lyrical content. The accompanying video (ムービー) was produced by yksb[KAZE Lab] and ハグキング. On the charting side, the Expert difficulty was designed by 鳩ホルダー (Hato Holder) and the Master difficulty by サファ太 (Safata). The Master chart sits at Lv14 with a difficulty constant of 14.0, featuring a dense note count of 781 (including 60 Touch notes and 78 Break notes) that places it among the upper tier of PRiSM PLUS original charts. 砂の函 was initially unlockable in 7sRef Area 4 by accumulating 450 km of travel distance. It received a general unlock on March 19, 2026 with the CiRCLE PLUS update. In the Asian version, the unlock became available on November 14, 2025. The song is available across all regions (Japan, International, USA).