The Beneath is hot! The temperature starts at 20°C at the very top, and only gets hotter as you descend.
Burlap, Linen, and Silk clothing will help, but here are some other ways to stay cool!
Water can be found all over the upper levels of the Beneath. Taking a quick dip can cool you off fast!
Bring along some Ice! Standing near it will lower the ambient temperature. Ice can be harvested on the surface using a Saw, and the resulting Ice Shavings can be turned into Packed Ice. Packed and Blue ice won't melt.
Ahhhh, so refreshing! Cools you down by 4°C.
Looking for something more permanent? The Blue Steel Diving Set will drastically lower your temperature, and lets you swim in lava too!
Lavaproof Hides can be obtained from the native Red Elk or looted from Bastions.