This is stage 2-8, the eighth level in the Migen's Shrine region.
Marina finds herself suddenly at the top of a long, tubular pit, with only several breakable brown blocks preventing her from hurtling downward.
In order to complete the stage, you must find and grab the exit warpstar that enclosed in a separate room at the bottom of the area.
When Marina first begins to fall, go straight down the middle, then take a right on the fork. There will be a break in the chain of spikeballs, and if she passes through this, and then moves immediately to the left, the golden gem will be against the wall of blue blocks. If the gem is missed while falling she can jump back up the wall along the ledges, expending health along the spikeballs, or reach the very bottom and start the level again from the top.