The impending crash.
Shallow water will not rescue unfortunate wanderers from being submerged and suffocated in the continuous flow of the roaring sea. Occupied only by a dense, black liquid, Level 405's vehement currents are powerful enough to drown even the most experienced swimmers. Though the fluid is initially only a meter deep, with every passing second, it appears as though the ground is sinking further into oblivion, leaving wanderers to silently struggle for their lives in the deep depths of the abyss. The liquid's high viscosity only complicates attempts at survival by actively inhibiting movement—wanderers may find themselves stuck in the sludge, immobilized as the fluid completely envelops and smothers victims.
A giant tidal wave, usually the final phenomenon one's eye fixates on before it closes forever, will conveniently be situated near the horizon, gradually approaching and growing like a tsunami—seemingly in all directions. Combined with the sheer lack of mobility, the tidal effects of Level 405 become a death sentence for helpless wanderers as they eagerly await their imminent destruction. The force of each wave that hits you rivals that of a hammer's swing; if asphyxiation fails to end a wanderer's life, a blunt impact to the head will promptly finish the job.
Repeated efforts to escape the inevitable will only accelerate one's demise. There is no hope in journeying further and exhausting the body's remaining energy; such an endeavor will cause wanderers to succumb rapidly and painfully. If one is stubborn enough to fight a losing battle, a floatation device is recommended to have even a minuscule chance of survival. However, in most cases, commitment to escape falters within the first few minutes of entering Level 405, and given enough time, wanderers will frequently gaze unperturbed at the oncoming waves to accept their brutal fate.