Minecraft wiki entry for Defense, associated text: With the introduction of the new water mechanics in 1.13, moats can be mixed with magma blocks to create an effective mob killing system, as bottom of the magma block will create whirlpool bubble columns that pull a mob to the bottom and then proceed to both pin them at the bottom and deal damage. If you wish to collect any drops created, use a hopper minecart on a powered rail one directly underneath the magma block to collect the drops and deposit them into a storage system.