Go into the save and hold w (no shift) while mining the obsidian. Once the obsidian is mined, the player will move fast enough with Depth Strider III to sink into the lava at the same time as the obsidian is formed, clipping into the obsidian block. You can now be completely trapped underneath obsidian inside a lava pit if you simply keep hold of w, or even worse press shift on reaction.

This may only work in my recreation attached, where there is a downward water flow but I have no time to test other cases. (Sorry for no video and only recreation instructions, I couldn't get OBS to work)