The Beneath is hot! The temperature starts at 15C at the very top, and only gets hotter as you descend.
Burlap 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 Packed and Blue ice won't melt.
Looking for something more permanent? The Blue Steel Diving Set will keep your temperature capped at 28C, and lets you swim in lava too!
Lavaproof Hides can be obtained from the native Red Elk or looted from Bastions.
Recipe: tfg:create/shaped/netherite_diving_helmet
Recipe: tfg:create/shaped/netherite_diving_helmet_upgrade
Recipe: tfg:create/shaped/netherite_backtank
Recipe: tfg:create/shaped/netherite_backtank_upgrade
Recipe: tfg:minecraft/shaped/netherite_leggings
Recipe: tfg:create/shaped/netherite_diving_boots
Recipe: tfg:create/shaped/netherite_diving_boots_upgrade