Maplestory "High Five" Private Showcase

Those hidden area mobs have way too much HP for the EXP they give.
Comparing the highest level mob in Lacheln:
Lv. 228
Max HP: 226,241,600
EXP: 363,430

And the typical hidden area mob:
Lv. 229
Max HP: 506,876,800
EXP: 402,466

That's a huge jump in HP for only a small boost to EXP, assuming you don't somehow 1hko these mobs.