Depression

YOU ARE IN A SMALL CHAMBER BENEATH A 3X3 STEEL GRATE TO THE SURFACE. A LOW CRAWL OVER COBBLES LEADS INWARD TO THE WEST. THE GRATE IS LOCKED.

From here:

You can go up outside the grate . Or to the cobble crawl. Or past the cobble crawl to the debris room. Or even further past the cobble crawl to a pit.