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KNOW IT ALL: Tuesday's Top Stories at NBC News

From the one-year anniversary of the Boston bombing to more dramatic testimony in the Pistroius trial, see the stories we're following.

Good morning, and happy Tax Day. Here are some of the stories we're following today:

1. Robotic submarine hunting for missing jet has big job ahead

The U.S. Navy-owned Bluefin-21, an unmanned sub, scanned the floor of the Indian Ocean for any signs of the missing Malaysian airliner. It was supposed to sweep for 16 hours, but a built-in safety feature ended the journey after it exceeded its “operation depth limit.” It’s a daunting mission that continues today. Read more in NEWS.

2. Tributes, emotions flood Boston one year after bombing

Boston will pause today to reflect and remember the horrific bombing that killed three people and injured more than 260 at last year’s Boston Marathon. A commemoration will take place at noon ET today, not far from the Boylston Street finish line where twin bombs were detonated. This year’s marathon will be held April 21. Read more in NEWS.

The outpouring of support has come from across the country:

3. Pistorius trial: Olympian was too ‘overcome with sadness’ to scream

Oscar Pistorius took the stand for a fifth and final day of cross-examination in the murder trial of girlfriend Reeva Steenkamp, who he claims he unintentionally shot. Prosecutor Gerrie Nel, who has been grilling the South African sprinter over his actions during the night Steenkamp died, asked Pistorius why he didn’t scream when he found he had shot her. “I don’t know what the purpose would be of screaming,” Pistorius replied. Read more in NEWS.

4. Ukraine launches anti-terror operation against pro-Russian separatists

Ukraine’s government in Kiev is looking to expel the separatists who have taken over government buildings in the country’s east. Russian officials, meanwhile, said they welcome potential dialogue between Ukraine and the pro-Russian separatists as a “step in the right direction.” Read more in NEWS.

5. Students return to site of Pennsylvania school stabbings

Franklin Regional High School in Murrysville will reopen to students today after last week’s stabbing rampage that injured 21 students and a security guard. The students will be allowed to walk the school with their families. The slow return to normalcy comes as new information released Monday shows that 16-year-old suspect Alex Hribal had made threats against two fellow students the night before the attack. Read more in NEWS.

6. Total lunar eclipse showcases ‘Blood Moon’ effect

If you happened to get up early today — and had clear skies above — you could have caught one spectacular sight: The first total lunar eclipse in more than two years. With the Earth positioned precisely between the sun and the full moon, the effect is a moon that gives off a brilliant reddish glow. Read more in TECH & SCIENCE.

… What’s trending today?

Vending machines these days dispense everything from iPads to cupcakes. And in Colorado, they’re offering pot products.

The maker of the ZaZZZ vending machine is setting up shop inside licensed dispensaries. Users need a driver’s license in order for the machine to verify their age.

“Almost like it’s a part of history,” Jerry Skinner, an investor from Massachusetts, told NBC affiliate KUSA in Denver. “If it works out, it could be a big thing.”