diskutil secureErase

macOS ships a multi-pass overwrite, and its own manual tells you not to trust it.

What it does

diskutil secureErase overwrites a whole disk, or just the free space on a mounted volume, with a choice of patterns:

diskutil list                              # identify the disk, carefully
diskutil secureErase 1 /dev/disk4          # single-pass random fill
diskutil secureErase freespace 0 /Volumes/Archive

Levels are 0 zero fill, 1 random fill, 2 seven passes, 3 the 35-pass Gutmann algorithm, 4 three passes. Levels 2 to 4 take an extremely long time and buy nothing on any drive made this century.

The disk argument is destroyed without a confirmation prompt, and a whole-disk erase leaves it unpartitioned and unusable until you partition it again. Check the identifier with diskutil list immediately beforehand — identifiers move when you plug something in.

Notes

Apple’s own manual page is blunt about the limits, and it is worth quoting rather than paraphrasing:

This kind of secure erase is no longer considered safe. Modern devices have wear-leveling, block-sparing, and possibly-persistent cache hardware, which cannot be completely erased by these commands. The modern solution for quickly and securely erasing your data is encryption.

So:

Internal storage on an Apple Silicon Mac is already encrypted by the Secure Enclave. System Settings → General → Transfer or Reset → Erase All Content and Settings discards the keys, which is instant and complete. That is the supported way to retire a Mac.
An external SSD is best encrypted from the day you start using it — then erasing the volume ends the story. Failing that, the drive’s own firmware sanitise command is the only thing that reaches spare blocks; see Parted Magic.
A mechanical disk has no wear levelling worth worrying about, and a single overwrite pass really is enough. This command is fine for those.

Alternative to

Alternative	Type	Trade-off
Erase All Content and Settings	Built in	Instant and complete, but only for the Mac’s own internal disk
ShredOS	Open source	Boots from a stick, wipes disks the running system is using, logs the result
Parted Magic	Paid	Issues the drive’s firmware sanitise command instead of overwriting it
dcfldd	Open source	Scriptable patterns and a hash of what was written
`shred` from coreutils	Open source	Per-file rather than per-disk, and just as defeated by wear levelling