Lava

Lava is perhaps one of the most difficult substances to work with in Terraria as it causes significant health damage upon impact (even thin lava puddles can do this). It begins to naturally occur around the depths of 1900ft to 2300ft. The only way to gather Obsidian is to allow water to touch lava. This can be difficult due to the lack of water in the -2100ft depths. Earthwelling is an effective yet difficult way to harvest vast amounts of obsidian however!