What you see: Black veins running through mango flesh
What it is: Darkened vascular canals!
Eat or toss? Cut around them if you’d like, but the mango is still fine to eat. 

The story: 

Perhaps one of the creepier mango defects, these black strings weird me out every time. But, thankfully the mango isn’t being overtaken by the narrow fingers of a shadow demon. Instead, it’s just sporting some darkened vascular canals. 

Even though the mango flesh looks like a solid entity, it’s actually laced with dozens of such vascular canals, which deliver sugars and other life-giving products of photosynthesis to the fruit. Kind of like our own blood vessels. 

The canals are normally the same color as the rest of the fruit’s interior. But circumstances can cause them to darken. For example, the hot water bath used to kill fruit flies, or too-cold storage can mess with the canals by disrupting cells and causing their contents to leak, commingle and then react to produce less than appealing colors. 

While here in the United States a reaction to an extreme temperature is the most likely explanation, Australian mango growers have been struggling with a similar issue, known as resin canal disorder, which was recently connected to a bacteria.

Whatever the cause, the mango, and the creepy lines, if you're inclined, are still fine to eat. 

SOURCES: 

Jeffrey Brecht. Post-harvest plant physiologist. Professor of horticultural science. University of Florida. 

Mango industry discovers highly infectious bacteria as cause of ugly veins of resin canal discolouration. By Matt Brann. 18 March 2019. ABC (Australia). 

Um, your canals are showing!