Thin Ice

It looks like a floor and it is not. Land on it at any real speed and it shatters, dropping you into whatever it was covering, usually a freezing pool in the middle of nowhere. The nastier half is that breaking through does not slow you down. Fall damage keeps building right through the break, so a long drop onto a sheet of it still hurts when you finally reach something solid. Cobwebs reset that counter; this does not.

• Ice Skates or anything built on them let you land without punching through.
• Anything that slows your descent works too: a Featherfall Potion, a tap of Rocket Boots, or falling through water first.
• It only gives way from above. You cannot run through it sideways or break up through the underside, and grappling to its base or its edges is safe. Grappling onto the top face is not.

Breaking it destroys it for good and drops nothing, so a snow biome you have run through a few times slowly loses its ice sheets. Water underneath keeps its level when the lid goes, so nothing floods. The large underground fields of it are a separate hazard, since crashing through several layers in a row can drop you a very long way.