From an Australian UXO tech sheet (https://www.defence.gov.au/sites/default/files/2025-03/Naval_marine-markers.pdf)
Both unexpended and expended markers are often found in maritime areas and washed ashore. A
number of incidents have occurred with the Mk 25 (and other markers) due to unexpended
phosphorus or a build-up of a phosphorous-type crust – this has occasionally resulted in either fire
injury/damage or low-order detonation (rupturing of the case) if the item is disturbed or re-exposed
to water. The marker also contains a small explosive charge which may cause injuries.
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