I first want to take a minute to recognize our people for how they've risen to the occasion over the past 12 months.
Now starting off on Slide 3 and 4.
There were a lot of highlights and accomplishments last year that led to adjusted EBITDA jumping over 50%, to $263 million and free cash flow climbing to $49 million.
Rochester finished the year much stronger than it started, with fourth quarter silver production increasing nearly 40%, and gold production up almost 50% quarter-over-quarter.
The updated mine plan reflects a reserves-only 18-year mine life with an NPV of $634 million and an anticipated IRR of 31%.
Production rates are also expected to double, driving average free cash flow to over $100 million per year.
During this time, we'll also remain focused on further expanding Rochester's silver and gold reserves beyond the 58% and 65% growth we saw last year.
Gold reserves grew by over 20%, and silver reserves increased by over 40% to the highest levels in Company history.
We've now dramatically increased our overall average mine life from just over seven years in 2015 to well over 12 years currently.
And with over $65 million allocated to exploration this year, we expect to see this number extend out even further.
We included several recent drill holes in yesterday's release from C-Horst, including one that was over 216 meters, averaging just about 1 gram per ton of oxide gold.
And we plan to invest approximately $10 million to continue growing this new discovery.
With only around half of the assays back at the end of the year, total resource tons increased over 40% and we more than tripled the strike length of the high-grade deposit to over 3.5 kilometers.
We plan to invest roughly $14 million in exploration at Silvertip this year, aimed at further expanding the resource and beginning to convert some of this material to reserves.
We also have identified and expect to lock down the flow sheet for a straightforward 1,750 ton a day process plant that can reliably deliver consistent recoveries and generate high-quality concentrates.
Before passing the call to Mick, I want to quickly highlight Slides 18 and 19 which provide a good high-level overview of our deep-rooted community relationships.
Now taking a look at Slide 6 and 7 and beginning with Palmarejo.
Strong results during the second half helped us finish the year on a high note, despite being down for roughly 45 days in the second quarter.
Together, these great accomplishments helped to generate nearly $93 million of free cash flow; Palmarejo's largest free cash flow year since 2017.
This will give us the opportunity to dial in the new unit before it goes into the expanded crusher corridor as part of POA 11.
The team's diligent focus and efforts helped us achieve our full year production and cost guidance, which led to a record $60 million of free cash flow.
Lastly, at Wharf, the team did a great job accomplishing their goals for the year, and achieved guidance by producing over 93,000 ounces of gold at an average cost around $890 per ounce.
More importantly, Wharf generated $73 million of free cash flow, shattering its previous record by over 25%.
Margin expansion from top line growth and prudent cost management helped us generate over $260 million in adjusted EBITDA and nearly $150 million in operating cash flow.
Both metrics were over 50% higher year over year.
These results showcase the power of our portfolio, especially during the second half of 2020 when our assets generated $86 million of free cash flow.
The strong second half more than offset the slower start to the year, leading to nearly $50 million of free cash flow in 2020, our highest annual figure since 2017.
I do want to flag that we are anticipating a relatively weaker first quarter, driven by, one, our mine plans, production profile and buildup of inventories on our leach pad; secondly, timing of tax payments in Mexico combined to be roughly $30 million to $35 million of cash outflow; and third, annual incentive payouts across the Company.
We bolstered our financial flexibility during the fourth quarter by fully repaying our revolving credit facility borrowings and expanding the capacity of the revolver to $300 million.
Together with our significantly improved cash position, this led to nearly $360 million of liquidity at the end of the year.
Particularly, our key leverage metric, net debt-to-EBITDA was cut in half year-over-year ending 2020 at 0.7 times.
We are targeting a net debt-to-EBITDA ratio of under 2 times, while maintaining at least $100 million of liquidity over the next two years as we complete major construction at Rochester.

Summary:
We are targeting a net debt-to-EBITDA ratio of under 2 times, while maintaining at least $100 million of liquidity over the next two years as we complete major construction at Rochester.