Mare Maris is an instrumental original composition by M2U (Shin Dong-hwi), a South Korean composer widely recognized in the rhythm-game community for his work across the DJMax series, Cytus, Deemo, and numerous Bemani titles. The track was created for maimai MiLK, where it additionally served as the game's ending theme, a rare role that gave the song extra prominence within the MiLK era. The song notably debuted not in maimai but in the location test for SEGA's sister rhythm game ONGEKI in December 2017, several months before arriving in maimai MiLK on May 22, 2018. Its initial availability in maimai was gated behind the Survival Course (the 6th term challenge track, titled 歴代チャレンジトラックランダムコース), followed by a brief period where it could be unlocked with 5 Cheese items, before becoming permanently available by default from the MiLK PLUS update onward. This multi-phase unlock path made Mare Maris a sought-after track during the early MiLK period. Musically, Mare Maris is a driving instrumental piece at 150 BPM, blending M2U's characteristic fusion of electronic synth textures with classical piano motifs. The track lacks lyrics entirely, letting the melody and rhythm carry the emotional arc. In maimai, its MASTER chart (level 13+, constant 13.9) is known for intricate simultaneous hold-note (EACH HOLD) patterns in the mid-section that demand precise finger independence, a design signature attributed to chart designer 小鳥遊さん (Takanashi-san). The EXPERT chart (level 12, constant 12.5) was designed by サファ太 (Safata). Over successive maimai versions, the chart levels of Mare Maris have been adjusted several times: its Advanced chart rose from 8 to 9 by FESTiVAL, Expert climbed from 10+ to 12, and Master settled at 13+ from Splash onward. The song's in-game genre label also shifted, originally categorized as ORIGINAL & JOYPOLIS in the MiLK through FiNALE era, it was relabeled to ORIGINAL in early でらっくす releases, and later simply as maimai from でらっくす PLUS onward.