window.dotcom = window.dotcom || { cmd: [] }; window.dotcom.ads = window.dotcom.ads || { resolves: {enabled: [], getAdTag: []}, enabled: () => new Promise(r => window.dotcom.ads.resolves.enabled.push(r)), getAdTag: () => new Promise(r => window.dotcom.ads.resolves.getAdTag.push(r)) }; setTimeout(() => { if(window.dotcom.ads.resolves){ window.dotcom.ads.resolves.enabled.forEach(r => r(false)); window.dotcom.ads.resolves.getAdTag.forEach(r => r("")); window.dotcom.ads.enabled = () => new Promise(r => r(false)); window.dotcom.ads.getAdTag = () => new Promise(r => r("")); console.error("NGAS load timeout"); } }, 5000)

Titanic sub: Why it's so hard to find

A search and rescue effort is under way in the North Atlantic to find a tourist submersible that went missing during a dive to the Titanic's wreck.

Alistair Greig, professor of marine engineering from University College London, explains why the search is co challenging.

By Griesham Taan & Melissa Hogenboom